NoSquint is a Firefox addon that “allows you to adjust the text-only and full-page (both text and images) zoom levels as well as color settings both globally (for all sites) and per site.” I have used it for years to make text larger, without also expanding everything else on the same page.
A friend recently complained about a website that used non-standard formatting, which made it harder for her to use. I looked at the site and agreed.
I found a couple of addons that changed or disabled some website formatting, NoSquint among them. Taking a new look at NoSquint, I saw that I could select text color, link color, and visited link color. What an improvement my selections made to that site’s usability!
A neat feature of NoSquint is that it can store your settings on a per-site basis, so changes you make for a site are necessarily applied universally. Also, you can select whether to make your selections permanent.
Note that this article is based on my use of Pale Moon. The NoSquint site comments seem to indicate that there may be issues with recent versions of Firefox.