Maya, Mari, Modo, Nuke use C++ Qt, Katana uses PyQt, other apps like Houdini use their own solution. And before someone says "that means they're not native" -

Maya, Mari, Modo, Nuke use C++ Qt, Katana uses PyQt, other apps like Houdini use their own solution. And before someone says "that means they're not native" - well obviously, and anyway, they have their own theme which is quite dark, as artists don't like bright colours in an interface distracting them from the content or what they're doing.