If you have a website, you can add a dark mode to it with this single #CSS rule :
html{
color-scheme: light dark
}
It will automatically adapt all background elements which don't have a background color already specified in the CSS, as well as the text color, to match the theme of the device! That's super useful if you spin up a quick project and don't want to spend a long time styling it! I use it on all of my new projects since I found this!