I use the built-in darkmode settings in qutebrowser [1], which work surprisingly well for most websites, but certainly not perfect. My impression is that the implementation is mainly in Qt, or Qt Webengine, so the same functionality may be available in other browsers with the same engine. The documentation also mentions Chromium, so perhaps equivalent settings are available there. (That said, qutebrowser is great.)

Anyway, my vague understanding of what is does is that it inverts all colors on the wrong side of a user-configurable brightness threshold (e.g. backgrounds that are too bright are inverted, and text that is too dark is inverted). This can make website colorschemes quite different, but most of the websites I visit regularly are still still readable, and often still reasonably nice-looking, despite having inverted colors.

The relevant part of my config file looks like this:

  # This requests the dark version of the website if it is available. Generally when this
  # works, I guess brightness-based the color inversion doesn't happen.
  c.colors.webpage.preferred_color_scheme = 'dark'
  c.colors.webpage.darkmode.algorithm = 'lightness-cielab'
  c.colors.webpage.darkmode.contrast = 0.0
  c.colors.webpage.darkmode.enabled = True
  c.colors.webpage.darkmode.grayscale.all = False
  c.colors.webpage.darkmode.grayscale.images = 0.0
  c.colors.webpage.darkmode.policy.images = 'smart'
  c.colors.webpage.darkmode.policy.page = 'smart'
  c.colors.webpage.darkmode.threshold.background = 128
  c.colors.webpage.darkmode.threshold.text = 128 

I haven't spent any time tweaking the threshold values, but maybe some tweaking would make it even better.

[1] https://qutebrowser.org/