Which property is the backgroundColor of a tab button?

[Tiemo Hollmann TB]

Hello,

I don't find the property to set the backgroundColor of a tab menu button
and its whole shape. If I change the backgroundColor of btn Tab Menu,
nothing changes, same with all other color properties.

The funny thing is, that the rectangle which sits below the TAB has a very
soft "white" color from the scratch, which gets darker to the bottom of the
rectangle. All patterns are also empty.

 

Thanks for any hint

Tiemo