Make an automatic dark theme variant of Planet GNOME using CSS "prefers-color-scheme" media query
It's mostly a matter of adding
@media (prefers-color-scheme: dark) {
/* all the properties/styles you want to apply to existing classes to make a dark variant */
}
The trickiest part would be the image gradients used by the speech bubbles containing the contents of each blog post, but other than that, it's mostly about saying "Text is white, hyperlinks are light blue, background is black or dark-gray" and you might already have an automatic dark theme on your hands that works automagically with Epiphany and Firefox that listen to GNOME 42's dark mode.