Sony's official Japanese site has updated with a new custom theme conversion utility. It allows users to create their own custom themes for firmware 3.70 and above.
The application does include an in-depth readme, detailing how to create new themes, but unfortunately it is entirely in Japanese. Those interested in dabbling with this new application can download it below, otherwise we'll be sure to let you know once Sony releases an English version of the utility.
Download Custom Theme Converter