Hi Tomas,
When DocumentManager’s is in SingleDocument view, NUIDocument resides in NSingleDocumentHost.
NSingleDocumentHost is ContainerControl which Controls[0] is the Panel that NUIDocument wraps.
To be able to make NSingleDocumentHost therefore NUIPanel transparent you should set its ControlStyle.SupportTransparentBackColor with conjunction with ControlStyle.UserPaint to true using SetStyle method in order to be able to set NSingleDocumentHost.BackColor to be transparent. Otherwise you will get an ArgumentException which said that control does not support transparent background color.
Sinse SetStyle is protected method, you should override it or call it in some place in the class, so you should create a new class that derives from NSignleDocumentHost, but this is not possible because NSingleDocumentHost is internal. So I am afraid that you cannot achieve the desired state.
Best Regards,
Nevron Support Team