The new GUI is darker for a couple of solid reasons, and I definitely could never stand to go back to the old one (which I've grown to hate). The reasons:
1. The new GUI is darker, which means that items, text, and similar on it "pops" compared to before. When reading text from characters, for instance, it's vastly more legible and the text looks inviting and bright.
2. This also works as a contrast point to the rest of the game, too. The menus themselves are dark, making them the negative space from an artistic standpoint. You aren't meant to be focusing on the menus; your eye is meant to be drawn to the world behind them! Of course when you need to navigate the menu you need to look at them, that's not what I mean -- but when you look at a picture that includes a menu, you shouldn't be going "look at that menu!" but instead going "look at that view!" Having the menus brighter than the world makes the world seem duller, or having them equal brightness just makes the menu get lost in the background more.
3. Also, speaking as a backdrop, for things like spell graphics, it's downright required in order to be able to see them clearly. Even when we had the old menus, we had to use darker menus for those kinds of screens. And so then we wound up with a really inconsistent set of menu graphics, where sometimes they would have a dark background (they had to), and where sometimes they had the inviting bright background (because it's nice and inviting). This was bad bad bad for a lot of reasons.
TLDR: It's not about the menus themselves, it's about how they fit into the overall aesthetic of the game, and what effect they have on your perception of the rest of what's going on with the screen. Taken on their own, I too probably prefer the old ones -- they do look cool and inviting. But actually in the game? I hate the old ones, in no uncertain terms.