Update app.py

app.py

@@ -2341,13 +2341,14 @@ SELECTION_JS = """
     }
     function toggleDark() {
         const url = new URL(window.location);
-        const isDark = url.searchParams.get('__theme') === 'dark';
+        const isBrowserDark = window.matchMedia && window.matchMedia('(prefers-color-scheme: dark)').matches;
+        const noTheme = !url.searchParams.has('__theme');
+        const isDark = noTheme ? isBrowserDark : url.searchParams.get('__theme') === 'dark';
         url.searchParams.set('__theme', isDark ? 'light' : 'dark');
         window.location.href = url.href;
     }
     window._lolaby_applyTheme = applyTheme;
     window._lolaby_toggleDark = toggleDark;
-
     window.addEventListener('load', function () {
         const url = new URL(window.location);
         const isDark = url.searchParams.get('__theme') === 'dark';
@@ -2358,7 +2359,7 @@ SELECTION_JS = """
             window.location.replace(url.href);
             return;
         }
-        applyTheme(isDark);
+        applyTheme(noTheme ? isBrowserDark : isDark);
     });
 })();