Have you tried non Bold'ing the topics but still keeping the different font color to distinguish between read and unread messages? It looks fine on the desktop, but slightly harder when utilizing a mobile.
It is possible, but 2 styles means 2 sets of files to maintain. It would be possible to switch based on some variable but it's kind of a complex adaptation for how the software is designed to work.
Have you considered a local css stylesheet that overrides the website settings? Is that possible with your mobile?