diff --git a/resources/views/partials/head.blade.php b/resources/views/partials/head.blade.php
index a4a43141da..c8fff4417f 100644
--- a/resources/views/partials/head.blade.php
+++ b/resources/views/partials/head.blade.php
@@ -47,6 +47,9 @@
 @endforeach
 
 <style>
+@if (\Statamic\CP\Color::isUsingThemeFile())
+    {{ \Statamic\CP\Color::themeCss() }}
+@else
     :root {
         {{ \Statamic\CP\Color::cssVariables() }}
 
@@ -54,6 +57,7 @@
             {{ \Statamic\CP\Color::cssVariables(dark: true) }}
         }
     }
+@endif
 </style>
 
 @stack('head')
diff --git a/src/CP/Color.php b/src/CP/Color.php
index 4758d73624..689b9d28f4 100644
--- a/src/CP/Color.php
+++ b/src/CP/Color.php
@@ -2,6 +2,8 @@
 
 namespace Statamic\CP;
 
+use Statamic\Facades\File;
+
 class Color
 {
     public const Slate = [
@@ -423,10 +425,14 @@ public static function defaults(bool $dark = false): array
             ->all();
     }
 
-    public static function theme(bool $dark = false): array
+    public static function theme(bool $dark = false): array|string
     {
         $config = config('statamic.cp.theme', []);
 
+        if (is_string($config) && File::exists(resource_path('themes/'.$config.'.css'))) {
+            return $config;
+        }
+
         foreach ($config[$dark ? 'dark-grays' : 'grays'] ?? [] as $shade => $value) {
             $colorHandle = $dark ? 'dark-gray' : 'gray';
             $config["{$colorHandle}-{$shade}"] = $value;
@@ -448,6 +454,16 @@ public static function theme(bool $dark = false): array
             ->all();
     }
 
+    public static function isUsingThemeFile(): bool
+    {
+        return is_string(static::theme()) && File::exists(resource_path('themes/'.static::theme().'.css'));
+    }
+
+    public static function themeCss(): string
+    {
+        return File::get(resource_path('themes/'.static::theme().'.css'));
+    }
+
     public static function cssVariables(bool $dark = false): string
     {
         return collect(static::theme($dark))