The readability unfortunately cant be fixed without a major design change (which I don't want to do), or going into about:config (which is scary).

By sheer coincidence there's a theme that came out before this one was big that's essentially exactly what you're asking for. Here's the link:

https://03.yfhome.xyz/en-US/firefox/addon/red-gaming-theme/?utm_source=03.yfhome.xyz&utm_medium=referral&utm_content=search