I currently have one script installed in my Tampermonkey browser extension that puts the forum on dark mode, I can't recall the thread where I saw and copy the code from, probably got it from the search option in TamperMonkey itself.
It can be done even easier with the BPIP extension to be installed on your browser (at least firefox or chrome based). And it comes with many other benefits (features).
But as said, this is a too often asked topic, just do a search please.