Lavandula,
Firefox has lots of themes that you can add on very easily by using the Tools on the menu bar and going to the Add-ons section.

here is the link to the themes and other add ons if you want to see what styles/colors there are-
https://addons.mozilla.org/en-US/fir...=100&sort=name

I also downloaded the nvidia driver program for my video card - with it I can really fine tune adjustments -the color/contrast/brightness/clarity of my display so it is easy on my eyes too. Much easier than adjusting with the monitor buttons.

you might have a different video card/brand but if you know the brand name you can go to the website and see if an update/upgrade is available.