At your own risk!

You only may modify SW (gupdate.gcd). In HEX editor you need to replace all string what contain:
0:/Themes/Map/
to
1:/Themes/Map/

Remember what 0:/Themes/Map/ is string in Unicode and you must find 0 : / T h e m e s / M a p / where instead "space" (x00 in HEX) you have use "wildcard"!

After that the Themes will work from a MemoryCard only with path mentioned above!