Though, it needs some preparations to have a small and clean QT install and be able to switch the theme.
I'm using the default/linux/amd64/10.0/desktop profile to set my global environment.
My USE-Flags in /etc/make.conf:
-qt4 -accessibility -plasma -jit -exceptions
USE-Flags in /etc/portage/package.use:
dev-python/PyQt4 -svg -webkit -multimedia -opengl
x11-libs/qt-gui -cups -mng -tiff gtkstyle
x11-libs/qt-sql -postgres -sqlite
It's important to enable qt3support, which is standard in the desktop profile. Then you won't need KDE's systemsettings because the qt3support package comes with a small tool called qtconfig to switch the theme. The 'GTK-alike' theme is built when you enable the gtkstyle USE-Flag.
All other USE-Flag should match your personal needs. I've disabled almost everything because I don't need support for most things in Picard or SMplayer.