Dark mode is basically changes the background color to dark color, the changes can be made as per your needs. This works on some simple javascript logic. Using the onclick event and adding custom css class using classList. The onclick button icon is from font-awesome offline mode. You can simply download the fontawesome offline file and link the main.css to get the full advantage of it or the icon will not be visible. Also add an image and some text which change color when switched to dark mode.