When inspecting the element, you can see:
font size ="-3"
for the micro-sized elements in HTML, with the features being inherited from the body text bootstrap.css:206 (I think?)
It actually looks good at font size="-1" when you manually edit it to see.
That is, readable, while still being distinct from the main body text which has font size set to 14px.
I believe "-3" means "set the size of this font to three sizes smaller than the body text". But reducing by 1 fixed size is more than enough to distinguish it from the main body.
Presumably that is some HTML setting set by the skin, and it's not manually configurable.
Or does it possibly relate to that .smallText setting in common.css that is set to font-size 66%?
If it can't be changed, that's a pity, because it looks too small right now.
I see the "Strapping" skin used by Whonix hasn't had any issues fixed on github since 2014. So, it's dead developer-wise.
Surely there's something better 3 years later that actually allows one to control basic elements like this in a config file e.g. maybe whatever Mozilla wiki is based off.