I had good results darkening leather with just water. Don't put it in a bowl, use a moist (but not dripping wet) cloth. The case should get darker instantly, and depending on type of leather and what it was initially treated with, chances are it will be slightly darker when it dries out than it was originally. You can do it on a small part of it to see if it brings any results. Repeat, but not too often, it could get too dry.
Also, sunlight works, even through window glass. Someone suggested mink oil, a very good idea and it actually conditions the case at the same time.