This is a simple web page that uses the theme color of your operating System and Apply's it to the background color.
https://color-pref.netlify.com/
This is a Css property called Prefers-Color-Scheme
as you can see the name is inside the media in the css links in the html.
👇
this then calls on to diffrent filse Dark or ligth, inside the css folder that you can find above the index.html file.
inside this to file you will only find one tag
:root
this tag controls what collors that are going to work when the dark theme is set or ligth theme inside your operating System
if you what to learn more you should take a look at this one: