style: Consolidate update banner styles and add theme support

Co-authored-by: aider (gemini/gemini-2.5-pro) <aider@aider.chat>

webclient/src/App.js

@@ -100,7 +100,7 @@ function App() {
 	return (
 		<div className={theme}>
 			{waitingWorker &&
-				<div style={{padding: 10, backgroundColor: '#ffffcc', color: '#000'}}>
+				<div className='update-banner'>
 					Client version mismatch, please refresh:{' '}
 					<button onClick={() => {
 						waitingWorker.postMessage({ type: 'SKIP_WAITING' });