I got stuck in the Soft Tabs tutorial

Hello everyone, I got stuck in a tutorial again.
Today it’s that with the tabs. Here is the link. Soft tabs - Composer since it would be a lot to describe, I took a lot of screenshots to describe it. Long story short, the background color doesn’t work. But why?

at first glance. it looks like tabs but it doesn’t work properly in the Composer Pro Canvas and in the MobileApp and Webview totally not.

first I created theme variables in “tabs”

Initially when the page is mounted, the PageVariable selectedTab is set to overview. That works.

I try to change the PageVariable with the tab and thus change the background. But what’s wrong here?
Thanks for your advice in advance
Jens