There used to be many white mice, but now that they don't blend in they are being eaten so a mutation occurred and made the color of the mice's fur to brown.
Now that the mice are being eaten so quickly that the predators don't have enough food to eat because they are reproducing to fast.
Since, the white mice are getting eaten they adapted so now their fur color is brown. The white mice aren't surviving long enough so they can't reproduce. There are more brown mice because they survive long enough to reproduce.
There are now more brown mice because the change in the gene pool. Even though there are more brown mice the gene of white fur will always be in the gene pool because every time there is a new gene the old one is being saved in the gene pool.