It's winter in Bucharest as of this writing, and heating is out. There's snow outside and we're expecting more. Might as well do something useful.
About this page
Basing a web page design on the browser's default colors cuts down on the amount of CSS required and gives you automatic dark mode. The flipside is eye-watering contrast with little to compensate for it. One idea would be to use some sort of gray outside the page borders (to soften the blow) but then you need a complete color scheme. Or is there a better way?
Turns out, one can use system colors and get the same effect without fuss. It's still good practice to break up the default white background somewhat, for example with headings or artwork.
How to use
The first line of CSS in the source of this page gives away the trick; the other nine are only there to make the page look like something. Note how you don't need to give any of the other colors like good practices say: in this case they're implicit. The browser should do the right thing even if it's been customized.
I don't think you need my permission to reproduce one line of code, or ten.