diff --git a/.changeset/static-markdown-theme-override.md b/.changeset/static-markdown-theme-override.md
new file mode 100644
index 000000000..f3879764d
--- /dev/null
+++ b/.changeset/static-markdown-theme-override.md
@@ -0,0 +1,5 @@
+---
+"gitbook": patch
+---
+
+Fix 404 on Markdown pages (View as Markdown) in site previews and when a theme is forced with `?theme=`.
diff --git a/packages/gitbook/e2e/internal.spec.ts b/packages/gitbook/e2e/internal.spec.ts
index 08605450c..f5846d320 100644
--- a/packages/gitbook/e2e/internal.spec.ts
+++ b/packages/gitbook/e2e/internal.spec.ts
@@ -1125,6 +1125,30 @@ const testCases: TestsCase[] = [
                     ).toBeVisible();
                 },
             },
+            {
+                name: 'Markdown page with theme cookie',
+                url: async () => {
+                    const data = await getSiteAPIToken(
+                        'https://gitbook.gitbook.io/test-gitbook-open/'
+                    );
+
+                    const searchParams = new URLSearchParams();
+                    searchParams.set('token', data.apiToken);
+                    searchParams.set('theme', 'dark');
+
+                    return `url/${getGitBookPreviewURL(`${data.site}/blocks?${searchParams.toString()}`)}`;
+                },
+                screenshot: false,
+                run: async (page) => {
+                    // The app opens previews with `?theme=`, which the middleware persists in a cookie.
+                    const markdownURL = new URL(page.url());
+                    markdownURL.pathname = `${markdownURL.pathname.replace(/\/$/, '')}.md`;
+                    markdownURL.search = '';
+                    const response = await page.request.get(markdownURL.toString());
+                    expect(response.status()).toBe(200);
+                    expect(response.headers()['content-type']).toContain('text/markdown');
+                },
+            },
             {
                 name: 'Redirect to app for authentication when missing token',
                 url: async () => {
diff --git a/packages/gitbook/src/middleware.ts b/packages/gitbook/src/middleware.ts
index 8a75df506..e346b9c8f 100644
--- a/packages/gitbook/src/middleware.ts
+++ b/packages/gitbook/src/middleware.ts
@@ -496,7 +496,9 @@ async function serveSiteRoutes(requestURL: URL, request: NextRequest) {
                     themeMode === CustomizationThemeMode.Dark
                         ? CustomizationDefaultThemeMode.Dark
                         : CustomizationDefaultThemeMode.Light;
-            } else {
+            } else if (routeTypeFromPathname !== 'static') {
+                // Markdown, llms.txt, RSS and sitemap routes only exist under `sites/static`, so
+                // forcing them dynamic 404s. They don't render a theme anyway. RND-12749.
                 routeType = 'dynamic';
                 requestHeaders.set(MiddlewareHeaders.Theme, themeMode);
             }