Actually the ContentPanel that appears in the tabstrip content area is a generic Windowsforms Panel control, and cannot be customized easily to display gradient background. That's why, by default, the content panel is transparent so it just displays the TabBase-gradient as a background. And you can customize the TabBase-fill element with the VSB application. If you need per-tabItem background customization, you can do it in the code-behind, handling the TabSelected event. You can access all the features of the gradient-fill primitive, like BackColor2, GradientStyle, etc, through the radTabStrip.TabElement.TabContent
property. Hope you'll have fun customizing the control as you wish.