diff --git a/polygerrit-ui/app/elements/plugins/gr-plugin-host/gr-plugin-host.js b/polygerrit-ui/app/elements/plugins/gr-plugin-host/gr-plugin-host.js
index b75ae44bf1..7476637016 100644
--- a/polygerrit-ui/app/elements/plugins/gr-plugin-host/gr-plugin-host.js
+++ b/polygerrit-ui/app/elements/plugins/gr-plugin-host/gr-plugin-host.js
@@ -40,7 +40,7 @@
           this._handleMigrations(plugins.js_resource_paths || [], htmlPlugins)
           .map(p => this._urlFor(p))
           .filter(p => !Gerrit._isPluginPreloaded(p));
-      const defaultTheme = config.default_theme;
+      const defaultTheme = this._urlFor(config.default_theme);
       const pluginsPending =
           [].concat(jsPlugins, htmlPlugins, defaultTheme || []);
       Gerrit._setPluginsPending(pluginsPending);
@@ -98,6 +98,9 @@
     },
 
     _urlFor(pathOrUrl) {
+      if (!pathOrUrl) {
+        return pathOrUrl;
+      }
       if (pathOrUrl.startsWith('preloaded:') ||
           pathOrUrl.startsWith('http')) {
         // Plugins are loaded from another domain or preloaded.
diff --git a/polygerrit-ui/app/elements/plugins/gr-plugin-host/gr-plugin-host_test.html b/polygerrit-ui/app/elements/plugins/gr-plugin-host/gr-plugin-host_test.html
index 641a800166..26958eeea7 100644
--- a/polygerrit-ui/app/elements/plugins/gr-plugin-host/gr-plugin-host_test.html
+++ b/polygerrit-ui/app/elements/plugins/gr-plugin-host/gr-plugin-host_test.html
@@ -188,6 +188,15 @@ limitations under the License.
       assert.equal(Gerrit._pluginInstallError.callCount, 2);
     });
 
+    test('default theme is loaded with html plugins', () => {
+      sandbox.stub(Gerrit, '_setPluginsPending');
+      element.config = {
+        default_theme: '/oof',
+        plugin: {},
+      };
+      assert.isTrue(Gerrit._setPluginsPending.calledWith([url + '/oof']));
+    });
+
     test('skips preloaded plugins', () => {
       sandbox.stub(Gerrit, '_isPluginPreloaded')
           .withArgs(url + '/plugins/foo/bar').returns(true)