You may want to check out mympx.org/forum to try and find themes or mods. Its a pretty broad forum, so you may have a bit of searching to do.
yeah i did search around a bit and found that spmp3052a had a frimware version that actually played other types than nes but cant seem to find a download link for that similar frimware version here is the text where they discuss this
Odd.I have this unit which uses the Sunplus spmp3052a
Same fw version 4.01.081017
Plays NES, GB, GBC and BIN games perfectly fine.Supposibly there is a fw upgrade to support SNES and GBA games but have not found it yet.
I've uploaded the firmware."
These players were popular btw the 2007-2009 era before the spmp8000 series came so most links are dead damn