Fix embed ignoring explicit ?theme override on single-theme sites (RND-11571)

resolveEmbeddableTheme checked the non-toggeable branch before the explicit
forcedTheme, so a site with the theme toggle disabled silently ignored the
embed's ?theme=light/dark (SDK colorScheme) request. Reorder so an explicit
override always wins, which is the only reliable way to force the scheme from
a mobile webview that can't run JS.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MN7HAu1vHzuFeND4UCRCBo

.changeset/rnd-11571-embed-force-theme.md

@@ -0,0 +1,5 @@
+---
+"gitbook": patch
+---
+
+Embed: an explicit `?theme=light` / `?theme=dark` (the SDK `colorScheme` option) now reliably forces the embed's color scheme, even on sites where the theme toggle is disabled. Previously single-theme sites ignored the requested scheme.

packages/gitbook/src/lib/embeddable.test.ts

@@ -146,14 +146,31 @@ describe('resolveEmbeddableTheme', () => {
         });
     });
 
-    it('keeps the site theme for single-theme sites', () => {
+    it('keeps the site theme for single-theme sites without an override', () => {
         expect(
             resolveEmbeddableTheme(
                 createCustomization({
                     toggeable: false,
                     default: CustomizationDefaultThemeMode.Light,
+                })
+            )
+        ).toEqual({
+            htmlTheme: CustomizationDefaultThemeMode.Light,
+            defaultTheme: CustomizationDefaultThemeMode.Light,
+            forcedTheme: CustomizationDefaultThemeMode.Light,
+        });
+    });
+
+    it('honors an explicit override on single-theme sites (RND-11571)', () => {
+        // A `?theme=light` embed on a site with the theme toggle disabled must still
+        // force the requested scheme, since a webview can only pass it via the URL.
+        expect(
+            resolveEmbeddableTheme(
+                createCustomization({
+                    toggeable: false,
+                    default: CustomizationDefaultThemeMode.Dark,
                 }),
-                CustomizationDefaultThemeMode.Dark
+                CustomizationDefaultThemeMode.Light
             )
         ).toEqual({
             htmlTheme: CustomizationDefaultThemeMode.Light,

packages/gitbook/src/lib/embeddable.ts

@@ -46,6 +46,18 @@ export function resolveEmbeddableTheme(
     customization: Pick<SiteCustomizationSettings, 'themes'>,
     forcedTheme?: CustomizationDefaultThemeMode | null
 ) {
+    // An explicit override (the embed's `?theme=` / `colorScheme` option) always wins, even for
+    // single-theme sites: the embedder is deliberately matching the color scheme of their own page,
+    // and a webview can only pass it via the URL. This must be checked before the toggeable branch,
+    // otherwise a site with the theme toggle disabled silently ignores the requested scheme. RND-11571
+    if (forcedTheme) {
+        return {
+            htmlTheme: forcedTheme,
+            defaultTheme: forcedTheme,
+            forcedTheme,
+        };
+    }
+
     if (!customization.themes.toggeable) {
         const mode = customization.themes.default;
         return {
@@ -56,14 +68,6 @@ export function resolveEmbeddableTheme(
         };
     }
 
-    if (forcedTheme) {
-        return {
-            htmlTheme: forcedTheme,
-            defaultTheme: forcedTheme,
-            forcedTheme,
-        };
-    }
-
     return {
         htmlTheme: CustomizationDefaultThemeMode.System,
         defaultTheme: CustomizationDefaultThemeMode.System,