Deluminate[0] for Chrome does a pretty good job of this. For some sites I find it does better than whatever themes happen to be available on userstyles.org. In other cases, it performs pretty poorly but it's still far better than having to hack together something myself for sites I care about.

[0] https://github.com/abstiles/deluminate

I liked this a lot, but it is way too heavy. Almost kills scrolling here, even on super-light sites as HN.

Interesting. I haven't experienced any noticeable performance issues at all. My biggest annoyance with it is that it sometimes make horizontal scrollbars appear when they wouldn't otherwise.

My computer is old.

On mobile, Firefox for Android has an neat addon for that:

Dark Background and Light Text by Mikhail Khvoinitsky https://addons.mozilla.org/en-GB/android/addon/dark-backgrou...

It just works™ about everywhere. It is also available on the desktop version, although I've never tried it myself.

For late night browsing/reading, I use flux's dark room mode, which turns everything into red and black.