diff --git a/packages/ckeditor5-core/package.json b/packages/ckeditor5-core/package.json
index bf81c8c27eb..576717abf15 100644
--- a/packages/ckeditor5-core/package.json
+++ b/packages/ckeditor5-core/package.json
@@ -62,6 +62,7 @@
     "lang",
     "src/**/*.js",
     "src/**/*.d.ts",
+    "theme",
     "CHANGELOG.md"
   ],
   "scripts": {
diff --git a/packages/ckeditor5-core/src/editor/editor.ts b/packages/ckeditor5-core/src/editor/editor.ts
index 0c0681f053d..1fabd0a2859 100644
--- a/packages/ckeditor5-core/src/editor/editor.ts
+++ b/packages/ckeditor5-core/src/editor/editor.ts
@@ -46,6 +46,8 @@ import { getEditorUsageData, type EditorUsageData } from './utils/editorusagedat
 import type { LoadedPlugins, PluginConstructor } from '../plugin.js';
 import type { EditorConfig } from './editorconfig.js';
 
+import '../../theme/core.css';
+
 declare global {
 	// eslint-disable-next-line no-var
 	var CKEDITOR_GLOBAL_LICENSE_KEY: string | undefined;
diff --git a/packages/ckeditor5-core/theme/core.css b/packages/ckeditor5-core/theme/core.css
new file mode 100644
index 00000000000..56746f4f929
--- /dev/null
+++ b/packages/ckeditor5-core/theme/core.css
@@ -0,0 +1,12 @@
+/*
+ * Copyright (c) 2003-2025, CKSource Holding sp. z o.o. All rights reserved.
+ * For licensing, see LICENSE.md or https://ckeditor.com/legal/ckeditor-licensing-options
+ */
+
+:root {
+	--ck-content-line-height: 1.5;
+}
+
+.ck-content {
+	line-height: var(--ck-content-line-height);
+}
diff --git a/packages/ckeditor5-icons/src/index.ts b/packages/ckeditor5-icons/src/index.ts
index 684f1c06dbb..abe99c19f28 100644
--- a/packages/ckeditor5-icons/src/index.ts
+++ b/packages/ckeditor5-icons/src/index.ts
@@ -80,6 +80,7 @@ export { default as IconIndent } from '../theme/icons/indent.svg';
 export { default as IconInsertMergeField } from '../theme/icons/insert-merge-field.svg';
 export { default as IconItalic } from '../theme/icons/italic.svg';
 export { default as IconLegalStyleList } from '../theme/icons/legal-style-list.svg';
+export { default as IconLineHeight } from '../theme/icons/line-height.svg';
 export { default as IconLink } from '../theme/icons/link.svg';
 export { default as IconListStyleCircle } from '../theme/icons/list-style-circle.svg';
 export { default as IconListStyleDecimalLeadingZero } from '../theme/icons/list-style-decimal-leading-zero.svg';
diff --git a/packages/ckeditor5-icons/theme/icons/line-height.svg b/packages/ckeditor5-icons/theme/icons/line-height.svg
new file mode 100644
index 00000000000..c09b4a4d46f
--- /dev/null
+++ b/packages/ckeditor5-icons/theme/icons/line-height.svg
@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 20 20"><path d="m17.668 15.096.698-.677.634.614L16.97 17l-1.967-1.967.634-.614.686.665V6.019l.001-.107-.69.669L15 5.967 16.97 4 19 5.967l-.634.614-.701-.68q.003.058.003.118zM1 4.722c0 .399.252.722.563.722h10.875a.5.5 0 0 0 .397-.211.84.84 0 0 0 .165-.51.84.84 0 0 0-.165-.511.5.5 0 0 0-.398-.212H1.563a.5.5 0 0 0-.398.212.84.84 0 0 0-.165.51m0 7.704c0 .399.252.722.563.722h10.875a.46.46 0 0 0 .215-.055.6.6 0 0 0 .182-.156.8.8 0 0 0 .122-.235.9.9 0 0 0 0-.553.8.8 0 0 0-.122-.234.6.6 0 0 0-.182-.156.46.46 0 0 0-.216-.055H1.563a.5.5 0 0 0-.398.211.84.84 0 0 0-.165.51m0 3.852c0 .398.252.722.563.722h7.446c.15 0 .293-.076.398-.212a.84.84 0 0 0 .165-.51.84.84 0 0 0-.165-.51.5.5 0 0 0-.398-.212H1.563a.5.5 0 0 0-.398.211.84.84 0 0 0-.165.51m0-7.704c0 .399.252.722.563.722h10.875a.5.5 0 0 0 .397-.211.84.84 0 0 0 .165-.51.84.84 0 0 0-.165-.512.5.5 0 0 0-.398-.211H1.563a.5.5 0 0 0-.398.211.84.84 0 0 0-.165.511"/></svg>
\ No newline at end of file