Remember how when Diablo III was first shown off, there was an uproar over how bright and cheery the game looked in comparison to its dark and brooding predecessors? Years later, some people just can't let that go.
There's a mod for those currently in the game's beta which lets them darken the colour scheme and lighting. If that's all it did I'd be inclined to call it a bit silly, but it actually sounds quite handy, as along with the darkness it also adds some post-processing effects similar to those found in a recent, awesome Skyrim fix, so it also makes everything look sharper as well.
You can find download links to the needed files below.