(This is mainly for Stack Overflow, I did personally not check out color schemes on the other sites. But a similar issue has been raised for beta sites.)
I don't think the current style for visited links on Stack Overflow posts is very nice. The text is
black and links are
#4A6B82 which looks almost like black or a very dark grey on my screen. That's not very accessible (and that's in spite of my good eyesight).
Additionally, links in inline code sections do not get underlined on hover, which makes them even harder to spot. Sometimes I link to documentation by putting a link on
ClassName(<- like this), and it is very hard to notice that it is even there (apparently, Meta has the same problem…).
I would find brighter links and consistent underlining very helpful.
I'll tag this as [bug] because I think link coloring is actually broken. And the other choices ([feature-request] [discussion] [support]) don't really fit.