1. Infra:
OCP/RH-SSO  sso76-openshift-rhel8:7.6-25 (RH-SSO 7.6.4)

2. Environment variable
oc set env dc/sso -e "JAVA_OPTS_APPEND=-Dkeycloak.profile=preview"
 

3. OCP RH-SSO pod log trace will start with 
7:34:04,516 INFO [org.jboss.as.server] (Controller Boot Thread) WFLYSRV0212: Resuming server
07:34:04,525 INFO [org.jboss.as] (Controller Boot Thread) WFLYSRV0025: WildFly Core 15.0.26.Final-redhat-00001 started in 31624ms - Started 726 of 1088 services (776 services are lazy, passive or on-demand)
07:34:04,529 INFO [org.jboss.as] (Controller Boot Thread) WFLYSRV0060: Http management interface listening on
http://127.0.0.1:9990/management
07:34:04,531 INFO [org.jboss.as] (Controller Boot Thread) WFLYSRV0054: Admin console is not enabled
07:35:45,772 ERROR [org.keycloak.theme.DefaultThemeManager] (default task-1) Failed to find ADMIN theme rh-sso.v2, using built-in themes
07:35:54,469 ERROR [org.keycloak.theme.DefaultThemeManager] (default task-4) Failed to find ADMIN theme rh-sso.v2, using built-in themes
07:35:55,444 ERROR [org.keycloak.theme.DefaultThemeManager] (default task-5) Failed to find ADMIN theme rh-sso.v2, using built-in themes

4) As a consequence the Rh-sso.v2 menu is only available for account, but not for for Login, Admin Console, and Email.

===> it makes the rh-sso.v2 theme selection unavailable when RH-SSOI is deployed on Openshift.