I recently bought a 21.5" iMac (late 2012). In the box containing the wireless keyboard and Magic Mouse was a white packet containing two Apple logo decals and many thin sheets of a dark-colored looking film. What is the dark-colored looking film for?

That dark-colored film is designed to clean the display of your iMac. Open it and use it to clean the display when necessary

Correct. You have to use it dry, without any liquid

You are welcome

The cleaning cloth in the transparent sleeve is a microfiber cloth just like is sold for cleaning eyeglasses. You would normally use it dry, as mende said, though if you have a stubborn bit of dirt on the screen you can dampen the cloth - and I do mean damp, not wet. Should you lose the cloth that came with the Mac, any normal eyeglass cloth will work as well.

Regards.