The only global setting I use is a 120% Full Zoom (I have cataracts and that helps me see without making it go all wonky). Right click on the magnifier symbol at the bottom right of the window to bring up the menu. Do not use Global Settings (Sorry, should have mentioned that, but I wasn't sure if it was just my computer acting wonky or something else).
Use Site Settings to set background and text colors. Note that even though I did not check the Disable background images box, there are sites that I can't see pictures on. On Ebay I can see thumbnails, but not the enlarged pictures. This site does not give me problems seeing pictures. Reaper is weird, I can see some big pictures, but not all of them.
Hopefully, this will help. If not, I think Chrome has a similar feature.