How to clean dirty magic keyboard

Remove the batteries. Dampen a lint-free cloth with clean, lukewarm water. Don't spray any type of cleaner on the cloth, as doing so may damage the keyboard. Wipe the surface of the keyboard gently with the damp cloth to remove dirt and stuck-on debris.

Hope this helped

I use a soft cloth slightly damp with Windex, have done this since my first Mac in 1984 to no ill effect. Still, simply damp with water will clean nearly as well, but not as fast. Also, I try to wipe it upside down as what is on the keys frequently either balls up or comes off in flakes. Wiping it upside down will help prevent any particulate from falling through the gaps into the interior of the keyboard.

How to clean your Apple products - Apple Support