If you want to create a website and want your visitor to view your site in dark mode or in different color, then this example is for you. I am sharing you the zip version of the example. Just download the zip and check the index.html file.
I have created the example with 2 style sheet. One for Light (white) version and another for Darker version. So, when you will visit the site for first time, it will be Light version and then if you click on the button, it will be darker version. Click Here To Download.