diff --git a/install/setup.sql b/install/setup.sql
index 18533f722d..bae0c3cb95 100755
--- a/install/setup.sql
+++ b/install/setup.sql
@@ -1102,6 +1102,7 @@ REPLACE INTO `{PREFIX}system_eventnames`
 ('214','OnManagerMenuPrerender','2',''),
 ('215','OnManagerTopPrerender','2',''),
 ('224','OnDocFormTemplateRender','1','Documents'),
+('225','OnBeforeMinifyCss', '1', ''),
 ('999','OnPageUnauthorized','1',''),
 ('1000','OnPageNotFound','1',''),
 ('1001','OnFileBrowserUpload','1','File Browser Events');
diff --git a/manager/includes/document.parser.class.inc.php b/manager/includes/document.parser.class.inc.php
index c73e063262..b3e60f605b 100755
--- a/manager/includes/document.parser.class.inc.php
+++ b/manager/includes/document.parser.class.inc.php
@@ -203,7 +203,7 @@ public function __construct()
 
         $this->q = self::_getCleanQueryString();
     }
-    
+
     final public function __clone()
     {
     }
@@ -5442,8 +5442,8 @@ public function invokeEvent($evtName, $extParams = array())
                 $this->pluginsCode .= '</fieldset><br />';
                 $this->pluginsTime["{$evtName} / {$pluginName}"] += $eventtime;
             }
-            if ($e->_output != '') {
-                $results[] = $e->_output;
+            if ($e->getOutput() != '') {
+                $results[] = $e->getOutput();
             }
             if ($e->_propagate != true) {
                 break;
@@ -6704,7 +6704,11 @@ class SystemEvent
 {
     public $name = '';
     public $_propagate = true;
-    public $_output = '';
+    /**
+     * @deprecated use setOutput(), getOutput()
+     * @var string
+     */
+    public $_output;
     public $activated = false;
     public $activePlugin = '';
     public $params = array();
@@ -6743,12 +6747,29 @@ public function alert($msg)
      * Output
      *
      * @param string $msg
+     * @deprecated see addOutput
      */
     public function output($msg)
     {
         $this->_output .= $msg;
     }
 
+    /**
+     * @param mixed $data
+     */
+    public function setOutput($data)
+    {
+        $this->_output = $data;
+    }
+
+    /**
+     * @return mixed
+     */
+    public function getOutput()
+    {
+        return $this->_output;
+    }
+
     /**
      * Stop event propogation
      */
@@ -6761,7 +6782,7 @@ public function _resetEventObject()
     {
         unset ($this->returnedValues);
         $this->name = "";
-        $this->_output = "";
+        $this->setOutput(null);
         $this->_propagate = true;
         $this->activated = false;
     }
diff --git a/manager/includes/header.inc.php b/manager/includes/header.inc.php
index 147c0f6e04..048f0a17e0 100755
--- a/manager/includes/header.inc.php
+++ b/manager/includes/header.inc.php
@@ -30,19 +30,36 @@
 if ($modx->config['manager_theme'] == 'default') {
     if (!file_exists(MODX_MANAGER_PATH . 'media/style/' . $modx->config['manager_theme'] . '/css/styles.min.css')
         && is_writable(MODX_MANAGER_PATH . 'media/style/' . $modx->config['manager_theme'] . '/css')) {
+        $files = array(
+            'bootstrap' => MODX_MANAGER_PATH . 'media/style/common/bootstrap/css/bootstrap.min.css',
+            'font-awesome' => MODX_MANAGER_PATH . 'media/style/common/font-awesome/css/font-awesome.min.css',
+            'fonts' => MODX_MANAGER_PATH . 'media/style/' . $modx->config['manager_theme'] . '/css/fonts.css',
+            'forms' => MODX_MANAGER_PATH . 'media/style/' . $modx->config['manager_theme'] . '/css/forms.css',
+            'mainmenu' => MODX_MANAGER_PATH . 'media/style/' . $modx->config['manager_theme'] . '/css/mainmenu.css',
+            'tree' => MODX_MANAGER_PATH . 'media/style/' . $modx->config['manager_theme'] . '/css/tree.css',
+            'custom' => MODX_MANAGER_PATH . 'media/style/' . $modx->config['manager_theme'] . '/css/custom.css',
+            'tabpane' => MODX_MANAGER_PATH . 'media/style/' . $modx->config['manager_theme'] . '/css/tabpane.css',
+            'contextmenu' => MODX_MANAGER_PATH . 'media/style/' . $modx->config['manager_theme'] . '/css/contextmenu.css',
+            'index' => MODX_MANAGER_PATH . 'media/style/' . $modx->config['manager_theme'] . '/css/index.css',
+            'main' => MODX_MANAGER_PATH . 'media/style/' . $modx->config['manager_theme'] . '/css/main.css'
+        );
+        $evtOut = $modx->invokeEvent('OnBeforeMinifyCss', array(
+            'files' => $files,
+            'source' => 'manager',
+            'theme' => $modx->config['manager_theme']
+        ));
+        switch (true) {
+            case empty($evtOut):
+            case is_array($evtOut) && count($evtOut) === 0:
+                break;
+            case is_array($evtOut) && count($evtOut) === 1:
+                $files = $evtOut[0];
+                break;
+            default:
+                $modx->webAlertAndQuit(sprintf($_lang['invalid_event_response'], 'OnBeforeMinifyManagerCss'));
+        }
         require_once MODX_BASE_PATH . 'assets/lib/Formatter/CSSMinify.php';
-        $minifier = new Formatter\CSSMinify();
-        $minifier->addFile(MODX_MANAGER_PATH . 'media/style/common/bootstrap/css/bootstrap.min.css');
-        $minifier->addFile(MODX_MANAGER_PATH . 'media/style/common/font-awesome/css/font-awesome.min.css');
-        $minifier->addFile(MODX_MANAGER_PATH . 'media/style/' . $modx->config['manager_theme'] . '/css/fonts.css');
-        $minifier->addFile(MODX_MANAGER_PATH . 'media/style/' . $modx->config['manager_theme'] . '/css/forms.css');
-        $minifier->addFile(MODX_MANAGER_PATH . 'media/style/' . $modx->config['manager_theme'] . '/css/mainmenu.css');
-        $minifier->addFile(MODX_MANAGER_PATH . 'media/style/' . $modx->config['manager_theme'] . '/css/tree.css');
-        $minifier->addFile(MODX_MANAGER_PATH . 'media/style/' . $modx->config['manager_theme'] . '/css/custom.css');
-        $minifier->addFile(MODX_MANAGER_PATH . 'media/style/' . $modx->config['manager_theme'] . '/css/tabpane.css');
-        $minifier->addFile(MODX_MANAGER_PATH . 'media/style/' . $modx->config['manager_theme'] . '/css/contextmenu.css');
-        $minifier->addFile(MODX_MANAGER_PATH . 'media/style/' . $modx->config['manager_theme'] . '/css/index.css');
-        $minifier->addFile(MODX_MANAGER_PATH . 'media/style/' . $modx->config['manager_theme'] . '/css/main.css');
+        $minifier = new Formatter\CSSMinify($files);
         $css = $minifier->minify();
         file_put_contents(
             MODX_MANAGER_PATH . 'media/style/' . $modx->config['manager_theme'] . '/css/styles.min.css',
diff --git a/manager/includes/lang/english.inc.php b/manager/includes/lang/english.inc.php
index 69432a70ad..3531f93db9 100755
--- a/manager/includes/lang/english.inc.php
+++ b/manager/includes/lang/english.inc.php
@@ -1503,3 +1503,5 @@
 $_lang['manager_menu_position_title'] = 'Main menu position';
 $_lang['manager_menu_position_top'] = 'top';
 $_lang['manager_menu_position_left'] = 'left';
+
+$_lang['invalid_event_response'] = 'The %s event has ivalid output';
diff --git a/manager/includes/lang/russian-UTF8.inc.php b/manager/includes/lang/russian-UTF8.inc.php
index 6789971195..1da7dd9fd0 100755
--- a/manager/includes/lang/russian-UTF8.inc.php
+++ b/manager/includes/lang/russian-UTF8.inc.php
@@ -1474,4 +1474,6 @@
 
 $_lang['manager_menu_position_title'] = 'Расположение основного меню';
 $_lang['manager_menu_position_top'] = 'вверху';
-$_lang['manager_menu_position_left'] = 'слева';
\ No newline at end of file
+$_lang['manager_menu_position_left'] = 'слева';
+
+$_lang['invalid_event_response'] = 'Событие %s вернуло неверный результат';
\ No newline at end of file