Transparent background for SplitApp and applications

Hi experts,

we are using UI5 version 1.22.3 and wonder if it is possible to make the background of the split app transparent , so that we can see the shell image through the split app?

In the former version we were able to achieve this via CSS, which seems to be not longer possible.

We also tried method SetBackgroundOpacity, but this only effects a background image of the SplitApp, which we don't use.

Our SplitApp uses it's default colors, no customizing here.

Example: the red area should be transparent so that we can see the background image with the flowers, which is the background image of the shell.

Any idea?

KR Steffen