That's the filtering. The underlined ones are the ones that people assigned to a category when they started the thread. When you click on it, you are presented with all threads of the same category.
As most people don't use them as they are (currently) optional most aren't underlined.
Use...
I over-wrote some graphics files by mistake.
What were in place where there are currently quite ugly envelopes on the main forum listing page? Was it less ugly envelopes or something else?