All available themes have the same color for visited and unvisited links

[ChangelingRain]

This is the default theme, and I've visited SCP-5000 and SCP-5002, which have very slightly darker link colors.

Sigma9, with no link color change

Night mode, with no link color change

Minimalist white, with no link color change

Matrix, with no link color change

Navy, with no link color change

Sandstone, with no link color change

Amber, with no link color change

Darkbody, with no link color change

Shark Punching Center, with no link color change

This is a bit of a problem, because I can't remember offhand which of several thousand SCPs I've already read, and it'd be nice if at least one of these themes marked visited links.