After looking at the "How to Free Up Memory" page, I noticed the following in my PC folder C:\Program Files\Common Files\Research In Motion\Shared\Loader Files\8320-v4.5.0.81_P2.7.0.78\Java
You can remove the carrier themes, but the stock BB Dimension themes must stay, as they are apart of the OS and can cause runnability issues if deleted...
Check the link in my sig (memory optimization) there is a section that talks about removing themes - just be careful in which you remove as the folks above you have stated.
Since you seem to be interested in gaining more memory the link will also help you in that regard.
You are literally talking about a few K of memory..........nothing that is affecting the operation of your BB, best advice like many have stated is to simply leave the themes on there.