It's very hard for an add-on to pin-point what to darken and what not, so most of these night-mode add ons, across all browsers don't look very nice.
To have a very well functioning night-mode add on it needs to be website specific, so you'd probably do better to do the following:
- Look for website enhancing and/or website specific night-mode add ons. (e.g. magic actions for youtube, reddit enhancement suite for reddit)
- Lower your monitor brightness, or use a brightness lowering overlay (e.g. f.lux or dimmer) if you simply want something more easy on your eyes.
Although not really an answer to your question, I hope this post can help you achieve some goal.