It's a blue t-shirt with the starfleet insignia on it!  There you go.  It isn't red, so you know that you won't get killed during the cold start to the episode and that's important.

Seriously, it is a bit snug for a large, so if, like me (and perhaps most Star Trek fans), you're a Clydesdale, you might want the XL.

Live long and prosper.