Fixes https://github.com/getgrav/grav/issues/476, replace default themes service with admin own theme service to avoid loading the default theme

admin.php

@@ -111,6 +111,12 @@ class AdminPlugin extends Plugin
             // Disable Asset pipelining
             $this->config->set('system.assets.css_pipeline', false);
             $this->config->set('system.assets.js_pipeline', false);
+
+            // Replace themes service with admin.
+            $this->grav['themes'] = function ($c) {
+                require_once __DIR__ . '/classes/themes.php';
+                return new Themes($this->grav);
+            };
         }
 
         // We need popularity no matter what

classes/themes.php

@@ -0,0 +1,18 @@
+<?php
+namespace Grav\Plugin;
+
+/**
+ * Admin theme object
+ *
+ * @author RocketTheme
+ * @license MIT
+ */
+class Themes extends \Grav\Common\Themes
+{
+    public function init()
+    {
+        /** @var Themes $themes */
+        $themes = $this->grav['themes'];
+        $themes->configure();
+    }
+}