fix(web): fix Theme Custom CSS endpoint requiring the user to be logged in as the server admin (#4633)

* fix custom css requiring the user to be the admin and logged in

* move theme api to custom endpoint

* add e2e test

web/src/routes/custom.css/+server.ts

@@ -1,6 +1,10 @@
 import { RequestHandler, text } from '@sveltejs/kit';
 export const GET = (async ({ locals: { api } }) => {
-  const { customCss } = await api.systemConfigApi.getConfig().then((res) => res.data.theme);
+  const {
+    data: {
+      theme: { customCss },
+    },
+  } = await api.serverInfoApi.getTheme();
   return text(customCss, {
     headers: {
       'Content-Type': 'text/css',