So my previous post disappeared. My userChrome.css changed the fonts in the inspector; my userContent.css changed fonts and font colors on the page, but as long as that section was limited to itch.io, it did not change the behavior of the nav section.
Viewing post in Game pages trigger my migraines
Well. As you can see, the position uses the attribute from game.css
At this point I would use standard bug search pattern.
Make backup of your userContent.css and try naked file with only the absolute important setting.
If this fails, try -p to start firefox with profile manager and make a fresh profile to try out. My best guess is, that you have some interfering addons or a } problem with your userContent.css. Syntax highlighting editors are good for that.