[ldd]

I replied to the top post, but just in case, here is my idea

It uses prefers-color-scheme and 28 lines of CSS:

https://pastebin.com/4JYbSi5F

[ldd]

And just in case 28 was too much, here's an improved version in 10 lines of CSS:

https://pastebin.com/dT4KKt3s