NDS:Original DS Screen Brightness Firmware Mod
As most owners of the original DS know, its backlight can be switched on or off, while the DS Lite and its successors replaced the feature with variable brightness levels (with the on/off feature baked into some early games leaving the not-anymore-reflective screens almost pitch black).
What's less known is that the last models of the NTR-001, including but not limited to the pink/light blue editions, use the same power management chip as the DS Lite and do support adjustable brightness (although the colors may look a bit washed out at the higher levels). These usually come with firmware 5.
Many flashcard kernels provide this feature natively, but the newest versions of NDS:FlashMe also add it to the regular main menu, just like on a DS Lite.