Ben
Yellowed plastic. That specific ivory-into-jaundice thing old keyboards do. It's not even dirt half the time, it's the plastic itself betraying its own polymers, UV eating the bromine out of the flame retardant and turning it into this sickly cream color like a cigarette-stained ceiling.
And the worst part is you can fix it with peroxide and sunlight, "retrobrighting," which means the ugliness was never necessary, just neglect wearing a costume of chemistry. I find that more annoying than if it were permanent.