Set default GTK theme if rc file doesn't exists.
See if default GTK theme is according to 'gsettings get org.gnome.desktop.interface gtk-theme'
You didn't delete the .pc/ directory so you need to get rid of that stuff (git rm), make a new commit, and then arc diff again.
I should point out that lintian warned you about that:
W: lxqt-config source: diff-contains-quilt-control-dir debian/.pc