Addresses #154 and finalize theme inheritance implementation

system/src/Grav/Common/Themes.php

@@ -27,6 +27,9 @@ class Themes extends Iterator
     {
         $this->grav = $grav;
         $this->config = $grav['config'];
+
+        // Register instance as autoloader for theme inheritance
+        spl_autoload_register([$this, 'autoloadTheme']);
     }
 
     public function init()
@@ -236,4 +239,35 @@ class Themes extends Iterator
             }
         }
     }
+
+    /**
+     * Autoload theme classes for inheritance
+     *
+     * @param  string $class Class name
+     *
+     * @return mixed  false  FALSE if unable to load $class; Class name if
+     *                       $class is successfully loaded
+     */
+    protected function autoloadTheme($class)
+    {
+        /** @var UniformResourceLocator $locator */
+        $locator = $this->grav['locator'];
+
+        $prefix = "Grav\\Theme";
+        if (false !== strpos($class, $prefix)) {
+            // Remove prefix from class
+            $class = substr($class, strlen($prefix));
+
+            // Replace namespace tokens to directory separators
+            $path = ltrim(preg_replace('#\\\|_(?!.+\\\)#', '/', $class), '/');
+            $file = $locator->findResource("themes://{$path}/{$path}.php");
+
+            // Load class
+            if (stream_resolve_include_path($file)) {
+              return include_once($file);
+            }
+        }
+
+        return false;
+    }
 }