https://github.com/darkreader/darkreader is great for this. Adds dark mode to any site.
It works to varying degrees of success depending on the site. Some sites become unreadable with it.
What I personally do is I have this flag enabled in my primary browser on my main computer, Brave on a MacBook Pro. And for any site that is severely broken I either skip that site, or if the site is important then I open the page in question in my secondary browser, Safari, and read it with its normal default style.