I really like the design of
M4Carbine.net. One reason I use TST and M4Carbine is because of the similar vBulletin layout. It's very easy to navigate.
Personally I like darker stylized themes. I know the desert digital pattern from M4Carbine wouldn't really fit with cars, but maybe a similarly laid out mix of carbon and black gloss with light/dark greys, maybe some sharp color accents of '00ff00' green or 'c80000' red... Nothing over the top, just everything in the right place to give the site a modern and customized feel. After all, that's what we're all here for. For me, TST is too old/generic looking. I think a site like this should have some pop to it, but stay sleek and classy at the same time.
Along with the new design, maybe revamp the logo as well. The solid black on white is great for decals and t-shirts, but again.. too generic for a modern car site. Same idea as the rest of the site... keep it simple and sleek, but just give it some depth. In my opinion, a properly done (not cheesy 3 min Photoshop tutorial) brushed metal texture or chrome would look great over a dark background.
I don't necessarily have all the answers or a perfect design in mind, these are just my observations and suggestions. TST is a well designed site overall, I just think it needs some modernization.