Implementation of dark mode

Dark mode is now enabled if the users device is set to use it.
Also minor stylistic changes were made

pages/login.html

@@ -19,7 +19,6 @@
       <input type="password" id="password" name="password" required />
       <button type="submit">Einloggen</button>
     </form>
-    <p id="errorMsg" style="color: red;"></p>
   </div>
   <!--<script src="../js/login.js"></script>-->
 </body>