I would use spray better than nail polish (carefully covering all the sensitive parts with film + tape). Maybe a Peppa Pig stencil and spray, in the black/grey background? Or spray all with silver and then black for the stencil.

Other possibility is to buy some nice, cheap stickers (like these ones), I found that they last quite long and when you want to change them, just take them off and clean with paint thinner. This way, she can be very elegant (black/grey is elegant, for ladies :) ) and change her laptop style quite often too.

An Edding paint marker in silver/gold colour can do the trick too (she can draw her own stuff in the cover, and it's clean easily with paint thinner too). Maybe you can find metallic purple or pink markers too.