In light of the recent firmware 3.80 release, Sony has updated its custom theme creation utility to version 1.2.0.0. The update is relatively minor, adding support for editing the Internet Radio icon found in 3.80.
Note that themes built with this version of the custom theme converter are not backwards compatible with firmware older than 3.80. Documentation on the utility can be found at the official PlayStation website.
Download Custom Theme Converter 1.2.0.0
PSP Custom Theme Tool [Sony]