Since Keycloak 12, it is no longer possible to use a custom theme from
a directory symbolic link, so make a raw copy of the swh theme in the
Keycloak theme folder instead.

For the record, I opened an issue on the subject in Keycloak JIRA
a couple of weeks ago and I got the following answer:

Rejecting this as a bug, as we want to make sure only files within the given 
directory can be used for security reasons. We never intended to support symlinks, 
which was a side-effect of a bug/vulnerability that was resolved.