Flashcart OS: Slot-1 Edition is a homebrew OS for Slot-1 Flashcarts. Current features include a boot menu, MoonShell booting, and boot to GBA.

Changes:

New features include skinning support! The only bug now is the no GBA booting when booted from the GnM. If you really need to try to boot GBA when used as bootme.nds, you can boot from DSOrganize. R4/M3 Simply skins may be used with little modification. See Skinning.txt in the ‘fshl’ directory.

http://forum.gbadev.org/viewtopic.php?t=14658