Problem UI Theme Designer and NWBC 5 for Desktop?

Hello,

I just started using UI Theme Designer. To demonstrate the Problem I just added an background image to a copy of the gold reflection theme.

After activating the theme in transaction /UI2/NWBC_CFG_CUST I started NWBC 4 for Desktop (PL 14) and everything is fine.

But the same user and system in NWBC 5 for Desktop looks like this:

Am I missing something or does there exist a problem  with using a custom theme in NWBC 5 for Desktop? (I have also tested it with a corbu theme, but it's the same.)

SAP_UI 740 SP-Level 12

Regards

Björn