feat: further reduce darkmode init flash (#5701)

* feat: further reduce darkmode init flash

* fix lint

* tweak doc

* colocate code

src/css/app.scss

@@ -34,6 +34,6 @@
 }
 
 .LoadingMessage--dark {
-  background-color: $oc-black;
-  color: $oc-white;
+  background-color: #121212;
+  color: #ced4da;
 }

src/excalidraw-app/index.tsx

@@ -524,6 +524,9 @@ const ExcalidrawWrapper = () => {
 
   useEffect(() => {
     localStorage.setItem(STORAGE_KEYS.LOCAL_STORAGE_THEME, theme);
+    // currently only used for body styling during init (see public/index.html),
+    // but may change in the future
+    document.documentElement.classList.toggle("dark", theme === THEME.DARK);
   }, [theme]);
 
   const onChange = (