Right now LXDE is written using the GTK2 tool-kit, but rather than transitioning to GTK3, it appears they will be using Qt. Right now there's an LXDE-Qt port in progress while for now the GTK2 version of LXDE is being maintained.
The LXDE-Qt port at the moment is targeting Qt4 but they intend to switch to Qt5 once the Qt 5.1 tool-kit is widely available within Linux distributions.
It appears the lightweight desktop developers behind LXDE are more fond of Qt4/Qt5 over GTK3. More details on the work-in-progress port can be found via their blog.