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">

settings_appearance.php

@@ -35,7 +35,7 @@
 </div>
 <div class="dark_theme_wrap" style="float:left;width:50%">
 <div class="switch">
-<input type="checkbox" id="setting_dark_theme" <?php if($_COOKIE['darktheme'] == "true") echo "checked='checked'" ?>>
+<input type="checkbox" id="setting_dark_theme" <?php if(array_key_exists('darktheme', $_COOKIE) && $_COOKIE['darktheme'] == "true") echo "checked='checked'" ?>>
 <div class="switch-btn">
 <span></span>
 </div>