In the right corner the next article is visible in the right corner, see here: https://tierarzthh.de/katzenkastration/
How can I get rid of that?
Please, go to Betheme -> Theme options -> Global -> Navigation & Share and uncheck the Sticky arrows.
Where can I set the bar
with the Tags and Categories over the article and also that kind of right sidebar like it is here?: https://themes.muffingroup.com/betheme/post-with-right-sidebar/
Where can I set the header (see attachment)?
Where can I set that header and the sidebar with that background colour?
The bar from your screenshot is a Subheader, which you probably have disabled in the Theme Options. Please, go to Betheme -> Theme Options -> Header & Subheader -> Subheader, and if you have Hide checkboxes selected, uncheck them.
To customize your sidebar, please, go to Betheme -> Theme Options -> Sidebars.
Thanks. But at Sidebars in Theme Options I cannot change the background colour...
Please, try the following code: