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

server/test/e2e/server-info.e2e-spec.ts

@@ -155,4 +155,16 @@ describe(`${ServerInfoController.name} (e2e)`, () => {
       });
     });
   });
+
+  describe('GET /server-info/theme', () => {
+    it('should respond with the server theme', async () => {
+      const { status, body } = await request(server).get('/server-info/theme');
+      expect(status).toBe(200);
+      expect(body).toEqual({
+        theme: {
+          customCss: '',
+        },
+      });
+    });
+  });
 });