Not sure if the discoloration can be reversed or evened out. If it can't be done, why not consider painting the steps (treads) a dark rich brown or even black, and paint the risers, spindles and trim a light complimentary color or white semigloss. :yes: Not sure what style the rest of your house is, but I like the look of painted steps I'm about to do mine too, my steps and spindles are stained a dark walnut but the look seems dated to me.
An example of painted treads.