I think there will be plenty of gloomy, gothic environments to suit most peoples taste. We've seen very little of the game so far and more colorful environments tend to play better for preview footage so it's no surprise they have been focusing on them.
I'm so tired of the copout answer of "Blizzard makes games that run on a wide variety blah blah blah"
It's not a copout answer though. You're assuming most PC gamers are desktop users but I would wager the majority are laptop users these days. Blizzard makes games for everyone, not just Bluesnews desktop gamer who has a dedicated graphics card.