Get PocketMac first, and then once you have that up and running:
1) Connect BB to Mac (>_<)
2) Click on the Blackberry icon
3) Select Utilities>Install App to Device
4) Select theme file
The only issue I've had with this is that my Curve will restart every single time I do this. Unless someone else has input on how to avoid this, we're just going to have to put up with that.
Either way, good luck on getting themes onto your BBs!