I have no additional themes. I think the risk you run is that they take a lot of space. If you run into space problems you risk losing critical data...phone logs, email, etc.
You should check your space Menu>options>status (if it's less than 16mb you really should consider deleting some or freeing space other ways. Here is a link that will help if you choose to free up memory. How do I free up memory on my device? - BlackBerryFAQ