I have dabbled extensively with themes and the worst that has happened is visual glitching. I've never heard of a theme bricking a berry.
If you purchase a theme Mobihand and BB Appworld are very safe vendors. Mobihand also gives you free backup copies and version upgrades.
Many free themes are available on Crackberry and the forums cover all issues associated with the particular theme.
Honestly, I've been far happier with the free themes rather than the few I have paid for.
But this forum has been the number one source of solving certain issues with the BB. However since the 5.0 experience I am happy to say my Storm has been running quite well. Be happy you came into the BB world around the 5.0 era. Let me tell you, I've been here since 18.104.22.168 and its been quite the uphill ride.
Welcome to the Blackberry experience brother.