Screenshots: https://imgur.com/a/1rVykMw
If you've never used a UserStyle before, use Stylus for it: https://github.com/openstyles/stylus#readme
It works on every browser that supports extensions I checked, though Android Firefox (Nightly) and things based on it will be a bit involved if you haven't set custom extensions yet. And noremal firefox only allows their set list.
After you got that installed just click on the raw file on the css and it'll prompt you to install it.
Or if you just want a direct link to mine:
https://github.com/Kurogal/Iwara-Black-Theme/raw/main/Iwara-Black-Theme.user.css
If Stylus itself is too blinding bright you should also use LunarTwilight's fork of the Stylus Dark theme. https://github.com/LunarTwilight/Stylus-Dark/blob/patch-1/stylus-dark.user.css Since the original by overdodactyl has an issue with a menu and hasn't been updated in over a year.