Hi Jeff

​If it is nicotine or tar on the keys, Scrubbing Bubbles® will lift it off. Lift the inner workings out of the plastic case so the keys and space bar are out in the open and set the machine in the kitchen sink. The keys must be completely dry before you start. Spray Scrubbing Bubbles® directly onto the keys and let soak for 10 to 30 seconds. If the white foam turns brown, you know it's working. Scrub the keys with a soft bristled dish brush and rinse off with hot water being careful not to get any water into the inner workings, then dry off with a soft cloth or towel.

​If this does not work, the plastic of the keys has probably oxidized due to the fire retardant 'bromine' in the plastic turning brown. If this is the case, you'll need to use a home made solution called Retro-Brite to actually reverse the molecular level change in the plastic's chemical make up. Do a Google search on "retrobrite" and all sorts of pages will come up giving you the recipe, method of use and length of time to keep the plastic parts in the solution. Hope this points you in the right direction,

​Sky

I would not have thought that it was nicotine - to deposit enough of it from the fingers to the keys to leave a visible stain would be nigh impossible.  Even folk who 'chew tobacco', and spit out a brown fluid regularly would not have it on their hands.