Design Time
You can access and modify the style for the different elements in the editable area of RadTabbedForm by using the Element hierarchy editor.
Programmatically
The following example demonstrates how you can access the visual item of the tab and change its back color.
Access the tab visual element
this.TabbedFormControl.Tabs[0].Item.BackColor = Color.Red;
Me.TabbedFormControl.Tabs(0).Item.BackColor = Color.Red