So breeze-dark turns into breeze_dark

Change-Id: Id5d06f73a79f099048b31fd16f9f9611236af392
Reviewed-on: https://gerrit.libreoffice.org/61321
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>