In order to change the default look of the tab panel, the tab’s css class property needs to be changed. The official AjaxToolkit site talks about how it should be done briefly:
http://www.asp.net/AJAX/AjaxControlToolkit/Samples/Tabs/Tabs.aspx
Here are some more extra resources found thru Google (duh!):
http://mattberseth.com/blog/2007/09/creating_a_yui_tabview_style_t.html
http://mattberseth.com/blog/2007/09/more_sample_ajaxcontroltoolkit.html
http://blogs.visoftinc.com/archive/2007/09/26/ajax-control-toolkit-tab-control-themes.aspx
By Bryan Xu