Added missing variables.less file

plugins/Login/Login.php

@@ -45,6 +45,7 @@ class Login extends \Piwik\Plugin
    public function getStylesheetFiles(&$stylesheetFiles)
     {
         $stylesheetFiles[] = "plugins/Login/stylesheets/login.less";
+        $stylesheetFiles[] = "plugins/Login/stylesheets/variables.less";
     }
 
     /**

plugins/Login/stylesheets/variables.less

+@login-section-background: #fafafa;