[BS5] add darkmode

data/web/templates/base.twig

@@ -9,6 +9,20 @@
   <title>{{ ui_texts.title_name|raw }}</title>
 
   <link rel="stylesheet" href="{{ css_path }}">
+  <script>
+    // check if darkmode is preferred by OS or set by localStorage
+    if (window.matchMedia && window.matchMedia('(prefers-color-scheme: dark)').matches ||
+        JSON.parse(localStorage.getItem("darkmode")) === true) {
+      var head  = document.getElementsByTagName('head')[0];
+      var link  = document.createElement('link');
+      link.id   = 'dark-mode-theme';
+      link.rel  = 'stylesheet';
+      link.type = 'text/css';
+      link.href = '/css/themes/mailcow-darkmode.css';
+      head.appendChild(link);
+    }
+  </script>
+
   <link rel="shortcut icon" href="/favicon.png" type="image/png">
   <link rel="icon" href="/favicon.png" type="image/png">
 </head>