From 03a75890dc282388dbf56777734735bdf619c65d Mon Sep 17 00:00:00 2001
From: Dmitrii Troitskii <jsleitor@gmail.com>
Date: Sun, 22 Feb 2026 03:18:48 +0000
Subject: [PATCH 1/5] feat: support custom shiki themes in code blocks

Add support for custom shiki theme objects (ThemeRegistrationAny-compatible)
in addition to built-in BundledTheme string names.

- Add CustomTheme interface and ThemeInput type alias
- Widen theme types across CodePluginOptions, CodeHighlighterPlugin,
  HighlightOptions, StreamdownProps, and StreamdownContextType
- Extract theme names from objects for cache keys and codeToTokens
- Pass theme objects to createHighlighter for registration
- Add tests for custom theme objects and mixed themes
- Full backward compatibility with existing BundledTheme usage

Closes #409
---
 .../streamdown-code/__tests__/index.test.ts   | 84 +++++++++++++++++++
 packages/streamdown-code/index.ts             | 57 +++++++++----
 packages/streamdown/index.tsx                 | 11 +--
 packages/streamdown/lib/plugin-types.ts       | 25 +++++-
 4 files changed, 156 insertions(+), 21 deletions(-)

diff --git a/packages/streamdown-code/__tests__/index.test.ts b/packages/streamdown-code/__tests__/index.test.ts
index b786d5c8..f175d132 100644
--- a/packages/streamdown-code/__tests__/index.test.ts
+++ b/packages/streamdown-code/__tests__/index.test.ts
@@ -1,5 +1,6 @@
 import type { BundledLanguage } from "shiki";
 import { describe, expect, it, vi } from "vitest";
+import type { CustomTheme } from "../index";
 import { code, createCodePlugin } from "../index";
 
 describe("code", () => {
@@ -303,4 +304,87 @@ describe("createCodePlugin", () => {
     expect(typeof plugin.getSupportedLanguages).toBe("function");
     expect(typeof plugin.getThemes).toBe("function");
   });
+
+  it("should create plugin with custom theme objects", () => {
+    const customLight: CustomTheme = {
+      name: "my-light-theme",
+      type: "light",
+      colors: { "editor.background": "#ffffff" },
+      tokenColors: [],
+    };
+    const customDark: CustomTheme = {
+      name: "my-dark-theme",
+      type: "dark",
+      colors: { "editor.background": "#1e1e1e" },
+      tokenColors: [],
+    };
+    const plugin = createCodePlugin({
+      themes: [customLight, customDark],
+    });
+    const themes = plugin.getThemes();
+    expect(themes[0]).toBe(customLight);
+    expect(themes[1]).toBe(customDark);
+  });
+
+  it("should create plugin with mixed built-in and custom themes", () => {
+    const customDark: CustomTheme = {
+      name: "my-dark-theme",
+      type: "dark",
+      colors: { "editor.background": "#1e1e1e" },
+      tokenColors: [],
+    };
+    const plugin = createCodePlugin({
+      themes: ["github-light", customDark],
+    });
+    const themes = plugin.getThemes();
+    expect(themes[0]).toBe("github-light");
+    expect(themes[1]).toBe(customDark);
+  });
+
+  it("should highlight code with custom theme objects", async () => {
+    const customLight: CustomTheme = {
+      name: "custom-light",
+      type: "light",
+      colors: {
+        "editor.background": "#ffffff",
+        "editor.foreground": "#000000",
+      },
+      tokenColors: [],
+    };
+    const customDark: CustomTheme = {
+      name: "custom-dark",
+      type: "dark",
+      colors: {
+        "editor.background": "#1e1e1e",
+        "editor.foreground": "#d4d4d4",
+      },
+      tokenColors: [],
+    };
+    const plugin = createCodePlugin({
+      themes: [customLight, customDark],
+    });
+
+    const callback = vi.fn();
+    const result = plugin.highlight(
+      {
+        code: "const x = 1;",
+        language: "javascript",
+        themes: [customLight, customDark],
+      },
+      callback
+    );
+
+    expect(result).toBeNull();
+
+    await vi.waitFor(
+      () => {
+        expect(callback).toHaveBeenCalled();
+      },
+      { timeout: 5000 }
+    );
+
+    const highlightResult = callback.mock.calls[0][0];
+    expect(highlightResult.tokens).toBeDefined();
+    expect(Array.isArray(highlightResult.tokens)).toBe(true);
+  });
 });
diff --git a/packages/streamdown-code/index.ts b/packages/streamdown-code/index.ts
index a459e283..d2475c33 100644
--- a/packages/streamdown-code/index.ts
+++ b/packages/streamdown-code/index.ts
@@ -14,6 +14,20 @@ import { createJavaScriptRegexEngine } from "shiki/engine/javascript";
 
 const jsEngine = createJavaScriptRegexEngine({ forgiving: true });
 
+/**
+ * A custom theme object compatible with shiki's ThemeRegistrationAny.
+ * Must have a `name` property for identification and caching.
+ */
+export interface CustomTheme {
+  name: string;
+  [key: string]: unknown;
+}
+
+/**
+ * Theme input type that accepts either a built-in theme name or a custom theme object.
+ */
+export type ThemeInput = BundledTheme | CustomTheme;
+
 /**
  * Result from code highlighting
  */
@@ -25,7 +39,7 @@ export type HighlightResult = TokensResult;
 export interface HighlightOptions {
   code: string;
   language: BundledLanguage;
-  themes: [string, string];
+  themes: [string, string] | [ThemeInput, ThemeInput];
 }
 
 /**
@@ -54,6 +68,14 @@ export interface CodeHighlighterPlugin {
    * Check if language is supported
    */
   supportsLanguage: (language: BundledLanguage) => boolean;
+  /**
+   * Get list of supported languages
+   */
+  getSupportedLanguages: () => BundledLanguage[];
+  /**
+   * Get the configured themes
+   */
+  getThemes: () => [ThemeInput, ThemeInput];
   type: "code-highlighter";
 }
 
@@ -63,9 +85,10 @@ export interface CodeHighlighterPlugin {
 export interface CodePluginOptions {
   /**
    * Default themes for syntax highlighting [light, dark]
+   * Accepts built-in theme names or custom theme objects.
    * @default ["github-light", "github-dark"]
    */
-  themes?: [BundledTheme, BundledTheme];
+  themes?: [ThemeInput, ThemeInput];
 }
 
 const languageAliases = Object.fromEntries(
@@ -104,10 +127,13 @@ const tokensCache = new Map<string, TokensResult>();
 // Subscribers for async token updates
 const subscribers = new Map<string, Set<(result: TokensResult) => void>>();
 
+const getThemeName = (theme: ThemeInput): string =>
+  typeof theme === "string" ? theme : theme.name;
+
 const getHighlighterCacheKey = (
   language: BundledLanguage,
-  themeNames: [string, string]
-) => `${language}-${themeNames[0]}-${themeNames[1]}`;
+  themes: [ThemeInput, ThemeInput]
+) => `${language}-${getThemeName(themes[0])}-${getThemeName(themes[1])}`;
 
 const getTokensCacheKey = (
   code: string,
@@ -121,9 +147,9 @@ const getTokensCacheKey = (
 
 const getHighlighter = (
   language: BundledLanguage,
-  themeNames: [string, string]
+  themes: [ThemeInput, ThemeInput]
 ): Promise<HighlighterGeneric<BundledLanguage, BundledTheme>> => {
-  const cacheKey = getHighlighterCacheKey(language, themeNames);
+  const cacheKey = getHighlighterCacheKey(language, themes);
 
   if (highlighterCache.has(cacheKey)) {
     return highlighterCache.get(cacheKey) as Promise<
@@ -132,7 +158,7 @@ const getHighlighter = (
   }
 
   const highlighterPromise = createHighlighter({
-    themes: themeNames,
+    themes: themes as (string | Record<string, unknown>)[],
     langs: [language],
     engine: jsEngine,
   });
@@ -147,7 +173,7 @@ const getHighlighter = (
 export function createCodePlugin(
   options: CodePluginOptions = {}
 ): CodeHighlighterPlugin {
-  const defaultThemes: [BundledTheme, BundledTheme] = options.themes ?? [
+  const defaultThemes: [ThemeInput, ThemeInput] = options.themes ?? [
     "github-light",
     "github-dark",
   ];
@@ -165,19 +191,23 @@ export function createCodePlugin(
       return Array.from(languageNames);
     },
 
-    getThemes(): [BundledTheme, BundledTheme] {
+    getThemes(): [ThemeInput, ThemeInput] {
       return defaultThemes;
     },
 
     highlight(
-      { code, language, themes: themeNames }: HighlightOptions,
+      { code, language, themes }: HighlightOptions,
       callback?: (result: HighlightResult) => void
     ): HighlightResult | null {
       const resolvedLanguage = normalizeLanguage(language);
+      const themeNames: [string, string] = [
+        getThemeName(themes[0]),
+        getThemeName(themes[1]),
+      ];
       const tokensCacheKey = getTokensCacheKey(
         code,
         resolvedLanguage,
-        themeNames as [string, string]
+        themeNames
       );
 
       // Return cached result if available
@@ -197,10 +227,7 @@ export function createCodePlugin(
       }
 
       // Start highlighting in background
-      getHighlighter(
-        resolvedLanguage as BundledLanguage,
-        themeNames as [string, string]
-      )
+      getHighlighter(resolvedLanguage as BundledLanguage, defaultThemes)
         .then((highlighter) => {
           const availableLangs = highlighter.getLoadedLanguages();
           const langToUse = (
diff --git a/packages/streamdown/index.tsx b/packages/streamdown/index.tsx
index 1f1d179e..20aeccd7 100644
--- a/packages/streamdown/index.tsx
+++ b/packages/streamdown/index.tsx
@@ -16,7 +16,6 @@ import rehypeRaw from "rehype-raw";
 import rehypeSanitize, { defaultSchema } from "rehype-sanitize";
 import remarkGfm from "remark-gfm";
 import remend, { type RemendOptions } from "remend";
-import type { BundledTheme } from "shiki";
 import type { Pluggable } from "unified";
 import { type AnimateOptions, createAnimatePlugin } from "./lib/animate";
 import { BlockIncompleteContext } from "./lib/block-incomplete-context";
@@ -25,7 +24,7 @@ import { hasIncompleteCodeFence, hasTable } from "./lib/incomplete-code-utils";
 import { Markdown, type Options } from "./lib/markdown";
 import { parseMarkdownIntoBlocks } from "./lib/parse-blocks";
 import { PluginContext } from "./lib/plugin-context";
-import type { PluginConfig } from "./lib/plugin-types";
+import type { PluginConfig, ThemeInput } from "./lib/plugin-types";
 import { preprocessCustomTags } from "./lib/preprocess-custom-tags";
 import { cn } from "./lib/utils";
 
@@ -45,10 +44,12 @@ export { parseMarkdownIntoBlocks } from "./lib/parse-blocks";
 export type {
   CjkPlugin,
   CodeHighlighterPlugin,
+  CustomTheme,
   DiagramPlugin,
   HighlightOptions,
   MathPlugin,
   PluginConfig,
+  ThemeInput,
 } from "./lib/plugin-types";
 
 // Patterns for HTML indentation normalization
@@ -128,7 +129,7 @@ export type StreamdownProps = Options & {
   /** Normalize HTML block indentation to prevent 4+ spaces being treated as code blocks. @default false */
   normalizeHtmlIndentation?: boolean;
   className?: string;
-  shikiTheme?: [BundledTheme, BundledTheme];
+  shikiTheme?: [ThemeInput, ThemeInput];
   mermaid?: MermaidOptions;
   controls?: ControlsConfig;
   isAnimating?: boolean;
@@ -179,12 +180,12 @@ const carets = {
 
 // Combined context for better performance - reduces React tree depth from 5 nested providers to 1
 export interface StreamdownContextType {
+  shikiTheme: [ThemeInput, ThemeInput];
   controls: ControlsConfig;
   isAnimating: boolean;
   linkSafety?: LinkSafetyConfig;
   mermaid?: MermaidOptions;
   mode: "static" | "streaming";
-  shikiTheme: [BundledTheme, BundledTheme];
 }
 
 const defaultStreamdownContext: StreamdownContextType = {
@@ -285,7 +286,7 @@ export const Block = memo(
 
 Block.displayName = "Block";
 
-const defaultShikiTheme: [BundledTheme, BundledTheme] = [
+const defaultShikiTheme: [ThemeInput, ThemeInput] = [
   "github-light",
   "github-dark",
 ];
diff --git a/packages/streamdown/lib/plugin-types.ts b/packages/streamdown/lib/plugin-types.ts
index e4a293f7..62ace602 100644
--- a/packages/streamdown/lib/plugin-types.ts
+++ b/packages/streamdown/lib/plugin-types.ts
@@ -1,5 +1,20 @@
 import type { MermaidConfig } from "mermaid";
 import type { BundledLanguage, BundledTheme } from "shiki";
+
+/**
+ * A custom theme object compatible with shiki's ThemeRegistrationAny.
+ * Must have a `name` property for identification and caching.
+ */
+export interface CustomTheme {
+  name: string;
+  [key: string]: unknown;
+}
+
+/**
+ * Theme input type that accepts either a built-in theme name or a custom theme object.
+ */
+export type ThemeInput = BundledTheme | CustomTheme;
+
 import type { Pluggable } from "unified";
 
 /**
@@ -30,7 +45,7 @@ export interface HighlightResult {
 export interface HighlightOptions {
   code: string;
   language: BundledLanguage;
-  themes: [string, string];
+  themes: [string, string] | [ThemeInput, ThemeInput];
 }
 
 /**
@@ -59,6 +74,14 @@ export interface CodeHighlighterPlugin {
    * Check if language is supported
    */
   supportsLanguage: (language: BundledLanguage) => boolean;
+  /**
+   * Get list of supported languages
+   */
+  getSupportedLanguages: () => BundledLanguage[];
+  /**
+   * Get the configured themes
+   */
+  getThemes: () => [ThemeInput, ThemeInput];
   type: "code-highlighter";
 }
 

From 77c649fc07b5e0bec15ecfdcdf236ea2064a3f9c Mon Sep 17 00:00:00 2001
From: Dmitrii Troitskii <jsleitor@gmail.com>
Date: Sun, 22 Feb 2026 10:10:11 +0000
Subject: [PATCH 2/5] fix: pass per-call themes to getHighlighter instead of
 defaultThemes

---
 packages/streamdown-code/index.ts | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/packages/streamdown-code/index.ts b/packages/streamdown-code/index.ts
index d2475c33..ecccba0f 100644
--- a/packages/streamdown-code/index.ts
+++ b/packages/streamdown-code/index.ts
@@ -227,7 +227,10 @@ export function createCodePlugin(
       }
 
       // Start highlighting in background
-      getHighlighter(resolvedLanguage as BundledLanguage, defaultThemes)
+      getHighlighter(
+        resolvedLanguage as BundledLanguage,
+        themes as [ThemeInput, ThemeInput]
+      )
         .then((highlighter) => {
           const availableLangs = highlighter.getLoadedLanguages();
           const langToUse = (

From a24ebbc9a5fe314fc23b7656cf332dff90434635 Mon Sep 17 00:00:00 2001
From: Dmitrii Troitskii <jsleitor@gmail.com>
Date: Sun, 22 Feb 2026 11:35:14 +0000
Subject: [PATCH 3/5] fix: resolve duplicate interface properties and type
 errors in CodeHighlighterPlugin

---
 packages/streamdown-code/index.ts       | 12 ++----------
 packages/streamdown/index.tsx           |  2 +-
 packages/streamdown/lib/plugin-types.ts | 10 +---------
 3 files changed, 4 insertions(+), 20 deletions(-)

diff --git a/packages/streamdown-code/index.ts b/packages/streamdown-code/index.ts
index ecccba0f..c57710ea 100644
--- a/packages/streamdown-code/index.ts
+++ b/packages/streamdown-code/index.ts
@@ -53,7 +53,7 @@ export interface CodeHighlighterPlugin {
   /**
    * Get the configured themes
    */
-  getThemes: () => [BundledTheme, BundledTheme];
+  getThemes: () => [ThemeInput, ThemeInput];
   /**
    * Highlight code and return tokens
    * Returns null if highlighting not ready yet (async loading)
@@ -68,14 +68,6 @@ export interface CodeHighlighterPlugin {
    * Check if language is supported
    */
   supportsLanguage: (language: BundledLanguage) => boolean;
-  /**
-   * Get list of supported languages
-   */
-  getSupportedLanguages: () => BundledLanguage[];
-  /**
-   * Get the configured themes
-   */
-  getThemes: () => [ThemeInput, ThemeInput];
   type: "code-highlighter";
 }
 
@@ -127,7 +119,7 @@ const tokensCache = new Map<string, TokensResult>();
 // Subscribers for async token updates
 const subscribers = new Map<string, Set<(result: TokensResult) => void>>();
 
-const getThemeName = (theme: ThemeInput): string =>
+const getThemeName = (theme: string | ThemeInput): string =>
   typeof theme === "string" ? theme : theme.name;
 
 const getHighlighterCacheKey = (
diff --git a/packages/streamdown/index.tsx b/packages/streamdown/index.tsx
index 20aeccd7..8d06756f 100644
--- a/packages/streamdown/index.tsx
+++ b/packages/streamdown/index.tsx
@@ -180,12 +180,12 @@ const carets = {
 
 // Combined context for better performance - reduces React tree depth from 5 nested providers to 1
 export interface StreamdownContextType {
-  shikiTheme: [ThemeInput, ThemeInput];
   controls: ControlsConfig;
   isAnimating: boolean;
   linkSafety?: LinkSafetyConfig;
   mermaid?: MermaidOptions;
   mode: "static" | "streaming";
+  shikiTheme: [ThemeInput, ThemeInput];
 }
 
 const defaultStreamdownContext: StreamdownContextType = {
diff --git a/packages/streamdown/lib/plugin-types.ts b/packages/streamdown/lib/plugin-types.ts
index 62ace602..57b6f86a 100644
--- a/packages/streamdown/lib/plugin-types.ts
+++ b/packages/streamdown/lib/plugin-types.ts
@@ -59,7 +59,7 @@ export interface CodeHighlighterPlugin {
   /**
    * Get the configured themes
    */
-  getThemes: () => [BundledTheme, BundledTheme];
+  getThemes: () => [ThemeInput, ThemeInput];
   /**
    * Highlight code and return tokens
    * Returns null if highlighting not ready yet (async loading)
@@ -74,14 +74,6 @@ export interface CodeHighlighterPlugin {
    * Check if language is supported
    */
   supportsLanguage: (language: BundledLanguage) => boolean;
-  /**
-   * Get list of supported languages
-   */
-  getSupportedLanguages: () => BundledLanguage[];
-  /**
-   * Get the configured themes
-   */
-  getThemes: () => [ThemeInput, ThemeInput];
   type: "code-highlighter";
 }
 

From 771dcd242360a853ddf547a23b0e9ed86d4aea22 Mon Sep 17 00:00:00 2001
From: Hayden Bleasel <hello@haydenbleasel.com>
Date: Tue, 3 Mar 2026 13:27:47 -0800
Subject: [PATCH 4/5] Add changeset for custom Shiki themes

---
 .changeset/custom-shiki-themes.md | 6 ++++++
 1 file changed, 6 insertions(+)
 create mode 100644 .changeset/custom-shiki-themes.md

diff --git a/.changeset/custom-shiki-themes.md b/.changeset/custom-shiki-themes.md
new file mode 100644
index 00000000..03780b6f
--- /dev/null
+++ b/.changeset/custom-shiki-themes.md
@@ -0,0 +1,6 @@
+---
+"streamdown": patch
+"streamdown-code": patch
+---
+
+Support custom Shiki themes in code blocks

From 60aba578e28ab304ba598404d1451b54c81b657e Mon Sep 17 00:00:00 2001
From: Hayden Bleasel <hello@haydenbleasel.com>
Date: Tue, 3 Mar 2026 14:35:44 -0800
Subject: [PATCH 5/5] fix: use shiki's ThemeRegistrationAny, deduplicate types,
 tighten API

- Replace custom CustomTheme interface with shiki's ThemeRegistrationAny
- Remove duplicated type definitions across packages
- Simplify HighlightOptions.themes to [ThemeInput, ThemeInput]
- Remove unsafe cast in createHighlighter call
- Bump changeset to minor (new feature, not patch)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
---
 .changeset/custom-shiki-themes.md             |  4 +--
 .../streamdown-code/__tests__/index.test.ts   | 13 ++++-----
 packages/streamdown-code/index.ts             | 29 +++++--------------
 packages/streamdown/index.tsx                 |  2 +-
 packages/streamdown/lib/plugin-types.ts       | 25 +++++-----------
 5 files changed, 24 insertions(+), 49 deletions(-)

diff --git a/.changeset/custom-shiki-themes.md b/.changeset/custom-shiki-themes.md
index 03780b6f..971a4624 100644
--- a/.changeset/custom-shiki-themes.md
+++ b/.changeset/custom-shiki-themes.md
@@ -1,6 +1,6 @@
 ---
-"streamdown": patch
-"streamdown-code": patch
+"streamdown": minor
+"streamdown-code": minor
 ---
 
 Support custom Shiki themes in code blocks
diff --git a/packages/streamdown-code/__tests__/index.test.ts b/packages/streamdown-code/__tests__/index.test.ts
index f175d132..e5dff7dc 100644
--- a/packages/streamdown-code/__tests__/index.test.ts
+++ b/packages/streamdown-code/__tests__/index.test.ts
@@ -1,6 +1,5 @@
-import type { BundledLanguage } from "shiki";
+import type { BundledLanguage, ThemeRegistrationAny } from "shiki";
 import { describe, expect, it, vi } from "vitest";
-import type { CustomTheme } from "../index";
 import { code, createCodePlugin } from "../index";
 
 describe("code", () => {
@@ -306,13 +305,13 @@ describe("createCodePlugin", () => {
   });
 
   it("should create plugin with custom theme objects", () => {
-    const customLight: CustomTheme = {
+    const customLight: ThemeRegistrationAny = {
       name: "my-light-theme",
       type: "light",
       colors: { "editor.background": "#ffffff" },
       tokenColors: [],
     };
-    const customDark: CustomTheme = {
+    const customDark: ThemeRegistrationAny = {
       name: "my-dark-theme",
       type: "dark",
       colors: { "editor.background": "#1e1e1e" },
@@ -327,7 +326,7 @@ describe("createCodePlugin", () => {
   });
 
   it("should create plugin with mixed built-in and custom themes", () => {
-    const customDark: CustomTheme = {
+    const customDark: ThemeRegistrationAny = {
       name: "my-dark-theme",
       type: "dark",
       colors: { "editor.background": "#1e1e1e" },
@@ -342,7 +341,7 @@ describe("createCodePlugin", () => {
   });
 
   it("should highlight code with custom theme objects", async () => {
-    const customLight: CustomTheme = {
+    const customLight: ThemeRegistrationAny = {
       name: "custom-light",
       type: "light",
       colors: {
@@ -351,7 +350,7 @@ describe("createCodePlugin", () => {
       },
       tokenColors: [],
     };
-    const customDark: CustomTheme = {
+    const customDark: ThemeRegistrationAny = {
       name: "custom-dark",
       type: "dark",
       colors: {
diff --git a/packages/streamdown-code/index.ts b/packages/streamdown-code/index.ts
index c57710ea..78d7a8c9 100644
--- a/packages/streamdown-code/index.ts
+++ b/packages/streamdown-code/index.ts
@@ -3,6 +3,7 @@
 import {
   type BundledLanguage,
   type BundledTheme,
+  type ThemeRegistrationAny,
   bundledLanguages,
   bundledLanguagesInfo,
   createHighlighter,
@@ -14,19 +15,7 @@ import { createJavaScriptRegexEngine } from "shiki/engine/javascript";
 
 const jsEngine = createJavaScriptRegexEngine({ forgiving: true });
 
-/**
- * A custom theme object compatible with shiki's ThemeRegistrationAny.
- * Must have a `name` property for identification and caching.
- */
-export interface CustomTheme {
-  name: string;
-  [key: string]: unknown;
-}
-
-/**
- * Theme input type that accepts either a built-in theme name or a custom theme object.
- */
-export type ThemeInput = BundledTheme | CustomTheme;
+export type ThemeInput = BundledTheme | ThemeRegistrationAny;
 
 /**
  * Result from code highlighting
@@ -39,7 +28,7 @@ export type HighlightResult = TokensResult;
 export interface HighlightOptions {
   code: string;
   language: BundledLanguage;
-  themes: [string, string] | [ThemeInput, ThemeInput];
+  themes: [ThemeInput, ThemeInput];
 }
 
 /**
@@ -77,7 +66,6 @@ export interface CodeHighlighterPlugin {
 export interface CodePluginOptions {
   /**
    * Default themes for syntax highlighting [light, dark]
-   * Accepts built-in theme names or custom theme objects.
    * @default ["github-light", "github-dark"]
    */
   themes?: [ThemeInput, ThemeInput];
@@ -119,8 +107,8 @@ const tokensCache = new Map<string, TokensResult>();
 // Subscribers for async token updates
 const subscribers = new Map<string, Set<(result: TokensResult) => void>>();
 
-const getThemeName = (theme: string | ThemeInput): string =>
-  typeof theme === "string" ? theme : theme.name;
+const getThemeName = (theme: ThemeInput): string =>
+  typeof theme === "string" ? theme : (theme.name ?? "custom");
 
 const getHighlighterCacheKey = (
   language: BundledLanguage,
@@ -150,7 +138,7 @@ const getHighlighter = (
   }
 
   const highlighterPromise = createHighlighter({
-    themes: themes as (string | Record<string, unknown>)[],
+    themes,
     langs: [language],
     engine: jsEngine,
   });
@@ -219,10 +207,7 @@ export function createCodePlugin(
       }
 
       // Start highlighting in background
-      getHighlighter(
-        resolvedLanguage as BundledLanguage,
-        themes as [ThemeInput, ThemeInput]
-      )
+      getHighlighter(resolvedLanguage as BundledLanguage, themes)
         .then((highlighter) => {
           const availableLangs = highlighter.getLoadedLanguages();
           const langToUse = (
diff --git a/packages/streamdown/index.tsx b/packages/streamdown/index.tsx
index 47508be8..e4633fe8 100644
--- a/packages/streamdown/index.tsx
+++ b/packages/streamdown/index.tsx
@@ -45,13 +45,13 @@ export { parseMarkdownIntoBlocks } from "./lib/parse-blocks";
 export type {
   CjkPlugin,
   CodeHighlighterPlugin,
-  CustomTheme,
   DiagramPlugin,
   HighlightOptions,
   MathPlugin,
   PluginConfig,
   ThemeInput,
 } from "./lib/plugin-types";
+export type { ThemeRegistrationAny } from "shiki";
 export {
   TableCopyDropdown,
   type TableCopyDropdownProps,
diff --git a/packages/streamdown/lib/plugin-types.ts b/packages/streamdown/lib/plugin-types.ts
index 57b6f86a..ea124186 100644
--- a/packages/streamdown/lib/plugin-types.ts
+++ b/packages/streamdown/lib/plugin-types.ts
@@ -1,22 +1,13 @@
 import type { MermaidConfig } from "mermaid";
-import type { BundledLanguage, BundledTheme } from "shiki";
-
-/**
- * A custom theme object compatible with shiki's ThemeRegistrationAny.
- * Must have a `name` property for identification and caching.
- */
-export interface CustomTheme {
-  name: string;
-  [key: string]: unknown;
-}
-
-/**
- * Theme input type that accepts either a built-in theme name or a custom theme object.
- */
-export type ThemeInput = BundledTheme | CustomTheme;
-
+import type {
+  BundledLanguage,
+  BundledTheme,
+  ThemeRegistrationAny,
+} from "shiki";
 import type { Pluggable } from "unified";
 
+export type ThemeInput = BundledTheme | ThemeRegistrationAny;
+
 /**
  * A single token in a highlighted line
  */
@@ -45,7 +36,7 @@ export interface HighlightResult {
 export interface HighlightOptions {
   code: string;
   language: BundledLanguage;
-  themes: [string, string] | [ThemeInput, ThemeInput];
+  themes: [ThemeInput, ThemeInput];
 }
 
 /**