its a bug of 3.71M33 which also likely in 3.60M33 slim...
probably happended while porting 1.5 recovery module to 3.xx.

using flash access thru recovery either corrupts the flash or it doesn't flash the file at all.

flashing files with a extended pandora menu with flash access is working like a charm. (yes I'm aware that there is nothing for slim yet ;p )

so advanced themes with prx aren't a problem at all ;)