Set default GTK theme if rc file doesn't exists.
Summary: Set default GTK theme if rc file doesn't exists.
Test Plan: See if default GTK theme is according to 'gsettings get org.gnome.desktop.interface gtk-theme'
Reviewers: wxl, tsimonq2
Reviewed By: wxl, tsimonq2
Differential Revision: https://phab.lubuntu.me/D27