userContent.css is frightfully inconvenient, since you have to restart the browser for it to take effect. I recommend (and use) the extension Stylus instead, which uses an inferior technique (the injected styles aren’t prioritised correctly as user styles), but can be tweaked more easily and as you go.
I usually test my preferences with Stylus, and once I found a combination I'm comfortable with, it gets perpetuated in the userContent.css for the technical reason you mentioned.
