I would say that you can't.
getComputedStyle(yourElement, '::selection').backgroundColor and
getComputedStyle(yourElement, '::-moz-selection').backgroundColor will return
transparent as default value and browser won't override os's default.
(Worth to be mentioned that if you set it to transparent, default os' value will be overriden).
I don't think browsers have access to os default preferences, and if they do, they probably won't let any website access it it so easily.