feat(web): custom stylesheets (#4602)

* add initial ui and api definitions for stylesheets

* proper saving

* make custom css work

* add textarea

* rebuild api

* run prettier

* add typecast

* update typings

* move css accordion to be sorted alphabetically

* set content-type properly

* rename stylesheets to theme

* fix server test

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

@@ -0,0 +1,9 @@
+import { RequestHandler, text } from '@sveltejs/kit';
+export const GET = (async ({ locals: { api } }) => {
+  const { customCss } = await api.systemConfigApi.getConfig().then((res) => res.data.theme);
+  return text(customCss, {
+    headers: {
+      'Content-Type': 'text/css',
+    },
+  });
+}) satisfies RequestHandler;