[FIX] Theme CSS loading in subdir env (#14015)

app/theme/server/server.js

@@ -9,6 +9,7 @@ import { Inject } from 'meteor/meteorhacks:inject-initial';
 
 import { settings } from '../../settings';
 import { Logger } from '../../logger';
+import { getURL } from '../../utils/lib/getURL';
 
 const logger = new Logger('rocketchat:theme', {
 	methods: {
@@ -78,10 +79,6 @@ export const theme = new class {
 			}
 			settings.updateById('css', data.css);
 
-			currentHash = crypto.createHash('sha1').update(data.css).digest('hex');
-			currentSize = data.css.length;
-			Inject.rawHead('css-theme', `<link rel="stylesheet" type="text/css" href="/theme.css?${ currentHash }">`);
-
 			return Meteor.startup(function() {
 				return Meteor.setTimeout(function() {
 					return process.emit('message', {
@@ -157,6 +154,12 @@ export const theme = new class {
 	}
 };
 
+settings.get('css', (key, value = '') => {
+	currentHash = crypto.createHash('sha1').update(value).digest('hex');
+	currentSize = value.length;
+	Inject.rawHead('css-theme', `<link rel="stylesheet" type="text/css" href="${ getURL(`/theme.css?${ currentHash }`) }">`);
+});
+
 WebApp.rawConnectHandlers.use(function(req, res, next) {
 	const path = req.url.split('?')[0];
 	const prefix = __meteor_runtime_config__.ROOT_URL_PATH_PREFIX || '';