MellyMel's "How-To" Guide for Blackberry

TO DELETE THEMES >> Go to Options/Advanced Options/Applications and scroll until you see the theme you want to delete under com_plazmic_theme_blah blah blah theme. Hit the menu button and select delete. It will prompt you to restart to save the changes.

I recommend doing this fairly often to prevent your device from getting laggy. I usually keep about 4-5 themes on my device but I know some people who can keep 8-10.