diff --git a/header.php b/header.php
index 3e3a4fe..29ea63b 100644
--- a/header.php
+++ b/header.php
@@ -13,7 +13,7 @@ include("language.php");
 <link rel="shortcut icon" href="/assets/images/favicon.ico">
 <link rel="gettext" type="text/x-gettext-translation" href="/locale/<?=$locale?>/LC_MESSAGES/messages.po">
 <link rel="stylesheet" href="/assets/css/style.css" media="all">
-<?php if($_COOKIE['darktheme'] == "true")
+<?php if(array_key_exists('darktheme', $_COOKIE) && $_COOKIE['darktheme'] == "true")
 echo '<link rel="stylesheet" href="/assets/css/dark.css" media="all">';
 ?>
 <link rel="stylesheet" href="/assets/css/fontawesome.min.css" media="all">