This is currently in work, covering most of the website's CSS.
TO INSTALL: You need to extract this in your chrome directory located at:
~/.mozilla/firefox/profile.default/chrome/userContent.css
Restart your Firefox and you are ready to go!
Edit the following in ~/.mozilla/firefox/profile.default/chrome/userContent.css
to your own liking:
:root {
--bg-color: #141414;
--fg-color: #9e9e9e;
--secondary-bg-color: #191919;
--secondary-fg-color: #616161;
--highlight-color: #752a2a;
--secondary-hl-color: #AC4142;
--third-hl-color: #90A959;
}