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://addons.mozilla.org/en-US/firefox/addon/red-gaming-theme/?utm_source=addons.mozilla.org&utm_medium=referral&utm_content=search

This was intentional. I've gotten an email asking for a white background, so I made a version with it and then switched it back to dark in the next version so people can choose which they want.

You should be able to fix this by either re-installing the theme, or by selecting the newest version on the sidebar