What you need: Hydrogen peroxide, baking soda and a rag or coarse sponge.
What you do: Sprinkle baking soda on the sheet, follow it up with some hydrogen peroxide and then sprinkle more baking soda on top. Let it sit for as long as two hours, then wipe it all away with a rag or coarse sponge. Voilà: good as new!
But what if I’m worried about all those chemicals being too harsh? Try it first on a tiny spot on the back of the sheet. No harm, no foul.