I’d be more interested in something that ensured that visited links were marked differently. Many, many sites go shushing up :visited when I would rather not have it shushed.
You could use one of the user stylesheet solutions to add your own styling. I like Stylebot, but I think it's only for Chrome.
Yeah - But I think you can do some of that stuff with the built-in Firefox Inspector. Just right-click and hit Inspect. On the right side of the toolbar click Style. You can try out styles there.
I’d like something more selective than that. Pages that don’t make them the same, do nothing; pages that make them the same, apply some algorithm to select a new colour which is likely to be appropriate in the context. Something like that.