I’ve used these few lines of CSS to apply dark mode to my website:

@media (prefers-color-scheme: dark) {
  body {
    filter: invert(100%);
    background-color: rgb(29, 32, 31) !important;
  }
  img,
  .astro-code,
  .emoji,
  iframe /* for recaptcha */ {
    filter: invert(100%) !important;
  }
}

Looks pretty good!

Lessons in this unit:

0:	Introduction
1:	How to center an element with CSS
2:	CSS Border inside the element
3:	What are good CSS Breakpoint values for Responsive Design?
4:	How to debug CSS by bisecting
5:	How to disable text selection using CSS
6:	How to put an item at the bottom of its container using CSS
7:	CSS, how to select elements that do NOT have a class
8:	How to stick an element on the bottom of the page with flexbox
9:	How to apply padding to multiple lines in CSS
10:	Making a table responsive using CSS
11:	CSS url()
12:	How to make an element smaller or bigger with CSS
13:	Customizing visited links
14:	Fix extra space after inline element
15:	How to create a sidebar that’s sticky but also scrolls
16:	How to embed YouTube videos using the correct aspect ratio
17:	Responsive pre tags in CSS
18:	Responsive YouTube Video Embeds
19:	How to remove all CSS from a page at once
20:	How I added Dark Mode to my website
21:	▶︎ How to add a simple dark mode