For what it’s worth, while I haven’t noticed this myself, I get the issue and see a good simple solution proposed.
(Stretching for reasons not to: The only reason I can see not to would be that arguably the darker text of that line creates more of a visual demarcation between posts. Though if it’s doing that, my inner UI geek screams that we’re deliberately obscuring that UI element for The Wrong Reason (dammit).)
So yeah, all good here.
We upgraded versions of Discourse today, and I see from elsewhere that peoples’ default theme settings were lost. Maybe the customisations went away, too?