You probably won’t be surprised that this can be done with a CSS customization 
The general approach to find these is to 
right-click the element you want to customize, select Inspect 
and look for the CCS classes 
. Then, you write CCS to target this (your favorite search engine will help you), and test it by directly injecting it in your browser 
.
Once you’re done, simply apply it in site settings.
In this case, the following will make these topics appear red:
.title.visited {
color: #FF0000 !important;
}
