Thanks, you led me towards the right direction 😃
I was trying it out on the Boost Campus theme so .label_content wasn't the correct class. After spending a couple of minutes inspecting the code, I found out it's under .no-overflow .
So in case anyone needs this on Boost Campus, please insert:
.no-overflow a {text-decoration: underline;}
It works, I'm so happy (also with the page activity)! Thanks again!
Kind regards
Tina