Use PSR-4 for plugin classes

classes/plugin/Themes.php

@@ -0,0 +1,22 @@
+<?php
+
+namespace Grav\Plugin\Admin;
+
+/**
+ * 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();
+        $themes->initTheme();
+
+        $this->grav->fireEvent('onAdminThemeInitialized');
+    }
+}