Right-click the desktop -> Personalize. It's one of the default themes.
I thought i'd seen a blue version of it that i liked, but unlike the other included wallpapers that one does not change color when you adjust the 2 color sliders. Too bad.
I ended up just using the bluish hot-air balloon wallpaper that is part of the "Lines" theme... or something like that. It's gorgeous, sharp, and above all: not distracting.