This guide is courtesy of PS3Mods.BlogSpot.com: Have you ever downloaded a PS3 theme, .p3t file, that you would like to customize? Some have icons you love but would like to use them with a different background.
This program unpacks PlayStation 3 Theme files (.p3t). By default, it will extract the contents of the theme file to the directory 'extracted' in the current directory.
IMPORTANT: You need to have the binary 'p3tcompiler.exe' in the same directory as this program, even when it is being run from Linux. This requirement will probably be removed soon.
This program is still in alpha stage and probably has dozens of bugs. If you find any, please let me know!