D9467: Properly check for Breeze GTK 3 theme existence in kconf_update

[Jack Gandy]

jackg accepted this revision.

REPOSITORY
  R98 Breeze for Gtk

BRANCH
  master

REVISION DETAIL
  https://phabricator.kde.org/D9467

To: fvogt, #plasma, jackg, apol
Cc: apol, plasma-devel, ZrenBot, progwolff, lesliezhai, ali-mohamed, 
jensreuterberg, abetts, sebas, mart