Description of problem: Docs  shows how to do in osp11 (appears to be copy of that version) . However in osp12 we use containers so the method is different. You will have to modify the horizon container and restart the process instead of the systemd service. Also will need to re-create the container image with the changes so they are persistent.
Version-Release number of selected component (if applicable):
This bugzilla has been removed from the release since it has not been triaged, and needs to be reviewed for targeting another release.
Another major issue is compared from OSP10, in the third step for osp12 all the paths are already relative, so no changes are necessary. If paths are changed in the custom them it will not work and break horizon!
Content is now live for OSP13 and OSP12: