Today Sony's official Japanese site (linked above) has released a new PSP Custom Theme Converter utility. It allows users to create custom themes for firmware 3.70 and above.
Download: PSP Custom Theme Converter
Included is a PDF file detailing how to create new themes, but unfortunately it is in Japanese. If anyone wishes to translate it post below, or check back (hopefully) for an English version of the utility at a later date.
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