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
"jbaconnh:
Odd.I have this unit which uses the Sunplus spmp3052a
DealExtreme sku#14785
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