If you have a need to apply custom styles to the AJAX Control Toolkit tab control you should take a look at Matt Berseth's post that includes examples.  You can also create your own styles....to do this, you can take a look at Matt's style sheets and then create your own from his. 

 In order to override the default styles, you will need to define the styles and then when you add the tab control, use the CssClass property to set the css style to use: