Add fallback to default skin if the selected one is not available in plugin directory

plugins/archive/archive.php

@@ -24,8 +24,8 @@ class archive extends rcube_plugin
     if ($rcmail->task == 'mail' && ($rcmail->action == '' || $rcmail->action == 'show')
       && ($archive_folder = $rcmail->config->get('archive_mbox'))) {
 
-      $skin_path = 'skins/'.$rcmail->output->config['skin'];
-
+      $skin_path = $this->local_skin_path();
+      
       $this->include_script('archive.js');
       $this->add_texts('localization', true);
       $this->add_button(

plugins/markasjunk/markasjunk.php

@@ -19,7 +19,7 @@ class markasjunk extends rcube_plugin
     
     $rcmail = rcmail::get_instance();
     if ($rcmail->action == '' || $rcmail->action == 'show') {
-      $skin_path = 'skins/'.$rcmail->output->config['skin'];
+      $skin_path = $this->local_skin_path();;
       $this->include_script('markasjunk.js');
       $this->add_texts('localization', true);
       $this->add_button(array(

program/include/rcube_plugin.php

@@ -231,6 +231,20 @@ abstract class rcube_plugin
     else
       return $fn;
   }
+  
+  /**
+   * Provide path to the currently selected skin folder within the plugin directory
+   * with a fallback to the default skin folder.
+   *
+   * @return string Skin path relative to plugins directory
+   */
+  protected function local_skin_path()
+  {
+      $skin_path = 'skins/'.$this->api->output->config['skin'];
+      if (!is_dir(realpath(slashify($this->home) . $skin_path)))
+        $skin_path = 'skins/default';
+    return $skin_path;
+  }
 
   /**
    * Callback function for array_map