Unfortunately, it is not possible to change the foreground of the TabItem because of the ContentPresenter that the Tab uses internally. The ContentPresenter itself does not expose a Foreground property.
Please find attached a project with a workaround. In the project I made a custom theme of TabItem control and replaced the ContentPresenter with ContentControl. You can edit the default ControlTemplates of the RadControl in Blend (read more). Also, please keep in mind that using the attached solution means that you will not be able to change the theme at Runtime.
If you have further questions please do not hesitate to ask us.
I hope this will help you.
the Telerik team
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