You can use dark reader and stylus (firefox extensions).
I use stylus to ‘correct dark reader’ or if i want write own style, or to change few elements on website.
I don’t remember how, but in dark reader in settings you can define your colors (background etc.), you can even set font! I use for example nerd font “CodeNewRoman NF” it works.
I hope after a year i will forget what reddit was.