Thank you for writing.
I will need more information about your scenario in order to provide you with proper support. Your screenshot suggests this is an autohidden ToolWindow with some dark blue element in it, however, I am not sure whether you would like make transparent the grey vertical strip (holding the Favorite tab) or the contents of the ToolWindow. In the former case please try the following approach:
RadPageViewStripElement stripElement =
stripElement.BackColor = Color.Transparent;
stripElement.BorderRightColor = Color.Transparent;
stripElement.BorderLeftColor = Color.Transparent;
stripElement.BorderTopColor = Color.Transparent;
The latter scenario requires more knowledge about the controls in your ToolWindow. Please provide further screenshots or code snippets.
I am looking forward to your reply.
the Telerik team