If you want to set the background of the content of the Pane you should set the background of its root element. The background of the Pane is used for the TabItem. You could use the following code:
In the code-behind you could find the Content first and set its Background.
Panes are added in the Items collection of the groups and if you want to insert a Pane you just need to insert it in the Items collection of the group in the index you wish. The AddItem method is used by the Drag&drop of panes to position a pane relatively to a group.
All the best,
the Telerik team
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