When loading a custom theme, the current configuration also loads the
StarlingX theme which is not available on the Openstack Horizon image.
This fix removes the loading of the StarlingX theme and fixes the logic
so other themes are enabled when the custom one is not.
Closes-Bug: #1935859
Signed-off-by: Thiago Brito <thiago.brito@windriver.com>
Change-Id: I14975ee07210875f0e5c49c13e3371a18c4b2261