if you are using tabcontainer control of ajax toolkit controls ,it has classes for hover,pressed and other cases for css ...
check this:
Tabs Css classes
.ajax__tab_header: A container element that wraps all of the tabs at the top of the TabContainer. Child CSS classes:.ajax__tab_outer.
.ajax__tab_outer: An outer element of a tab, often used to set the left-side background image of the tab.Child CSS classes: .ajax__tab_inner.
.ajax__tab_inner: An inner element of a tab, often used to set the right-side image of the tab. Child CSS classes:.ajax__tab_tab.
.ajax__tab_tab: An element of the tab that contains the text content. Child CSS classes:none.
.ajax__tab_body: A container element that wraps the area where a TabPanel is displayed. Child CSS classes: none.
.ajax__tab_hover . This is applied to a tab when the mouse is hovering over. Child CSS classes:.ajax__tab_outer.
.ajax__tab_active: This is applied to a tab when it is the currently selected tab. Child CSS classes:.ajax__tab_outer.
otherwise you can do this ...
for example your tab ID is "TabExample"
in you css file enter this :
#TabExample:activated
and this is for hover case:
#TabExample:hover
hope this solve your problem !