style: update Composer component background color for improved visibility

Author: Crokily

app/components/assistant-ui/thread.tsx

@@ -288,7 +288,7 @@ const Composer: FC<ComposerProps> = ({
   }, [onClearError, onOpenChange]);
 
   return (
-    <div className="aui-composer-wrapper sticky bottom-0 mx-auto flex w-full max-w-[var(--thread-max-width)] flex-col gap-4 overflow-visible rounded-t-3xl pb-4 md:pb-6">
+    <div className="aui-composer-wrapper sticky bottom-0 mx-auto flex w-full max-w-[var(--thread-max-width)] flex-col gap-4 overflow-visible rounded-t-3xl bg-white pb-4 md:pb-6">
       <ThreadScrollToBottom />
       <ThreadPrimitive.Empty>
         <ThreadWelcomeSuggestions />