Edit: my fault. I have too many dark mode addons/stylesheets enabled at the same time.
Midnight lizard (chrome) https://chrome.google.com/webstore/detail/midnight-lizard/pb... is a good extension that can create a dark theme by changing the contrast & things like that. It does it in a way that the same theme will work on a lot of sites. They do have more dark (& light) themes on their site & you can create your own by entering a few colors & it will create a theme based on that. Stylus is good (NOT stylish), but Midnight lizard creates themes in a way that are compatible with a lot more sites that Stylus.
None of the extensions I've tested can match the dead simple CSS override I used in qutebrowser.
old.reddit.com comments are mostly unreadable, and it cripples gmail, but I've learned to take advantage of Options-Shift-D to flip it on and off when needed.
It's now part of my web browsing workflow.
I second the other commenter, don't do anything. Or get someone really good to help you!