Summary: Can't change GTK3 theme in lxqt-config-appearance if settings don't exist, so create it. Test Plan: 1. Open lxqt-config-appearance 1. Check "Set GTK themes" 1. Change "GTK 3 Theme" to Arc 1. Close lxqt-config-appearance 1. Open lxqt-config-appearance "GTK 3 Theme" should be set to Arc and not the default, Adwaita. Reviewers: tsimonq2 Reviewed By: tsimonq2 Differential Revision: https://phab.lubuntu.me/D13