Thank you for the question.
I am not quite sure I understand your scenario completely. Every theme is made for a specific control and other controls cannot inherit it. Therefore, I am wondering how other controls can inherit your Outlook theme. Do you mean that the TabStrip does not look as in Outlook? Its default theme (ControlDefault theme) mimics Office 2007 Blue, so any additional details on your case will be welcome.
The theming of RadControls cannot be disabled. However, you can make a transparent theme for your RadTabStrip, so it will get the colors of the parent container (a form or a panel).
In order to make your transparent or semi-transparent theme, you should set an Alpha channel to the colors in the theme for the RadTabStrip's subelements. Generally, a color is defined by three integer numbers for the Red, Green and Blue colors. To set an Alpha channel, you should insert a forth integer number in front of these three numbers.
For example you have a color defined as 155, 155, 155
for the RGB standard. To see an Alpha channel of 100, you should set it as 100, 155, 155, 155
I am attaching an example of a semi-transparent RadTabStrip theme. If you have additional questions, feel free to contact me.
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