This is not control specific but rather dependent more upon how a form designer and tabs interact with one another.  But the problem in that example started after the custom user control was dropped on the form.

Just close the form designer and then open it back up.  That is all.  If you are getting some other error or if this doesn't resolve the problem then you are more than likely dealing with something else.

We use an Infragistics tab control and sub class it and then set the appropriate properties from the ApplicationTheme.ThemeProperties properties.  This way the tab will follow themes and the inherited tab control can just be dropped on any form from that point on.  This is one of the controls you would want to setup in your UI assembly that we talked about during class.

That TabControl is not Themed.  If you look in the docs it will give you a list of all of the controls (as does the training material in the APplicationThemes section) that have Theme support.  What we do is use the new ThemedLinkmenu control and a Panelmanager (we did this in the UI sample we created in class) and create a vertical page tab.

No.  This control will more than likely never be on the themed list as the standard .NET tab control has some inherant issues in order to do this.  At some point we will have a tab control, but may not happen for some time.