Qt5 does not seem to be a part of the Lisp world yet and it's unclear when that will happen, so I'm, more or less, stuck between two options:
The current Tk8.6 release does not seem to have wide support yet, I only found simple-tk to support it, but styling Tk's widgets feels weird to me, especially since its documentation doesn't even mention how I can do that.
It's good to know that there is working support for GTK+ 3, and while everything works like a charm on OS X, Windows has a "known issue" of freezing the complete environment when I try to move the window. The author seems to have disappeared by the end of 2014.
Can you help?