first, you need to make a 9 patch PNG with the border line at the bottom. The fastest way I know is through : http://romannurik.github.io/AndroidAssetStudio/nine-patches.html
Then you would make a selector xml on your drawable folder like this one tabs_backgrounds.xml :
<!-- Non focused states -->
<item android:drawable="@color/xlight_grey" android:state_focused="false" android:state_pressed="false" android:state_selected="false"/>
<item android:drawable="@color/white_back" android:state_focused="false" android:state_pressed="false" android:state_selected="true"/>
<!-- Focused states -->
<item android:drawable="@color/xlight_grey" android:state_focused="true" android:state_pressed="false" android:state_selected="false"/>
<item android:drawable="@color/white_back" android:state_focused="true" android:state_pressed="false" android:state_selected="true"/>
<!-- Pressed -->
<!-- Non focused states -->
<item android:drawable="@color/light_grey_color" android:state_focused="false" android:state_pressed="true" android:state_selected="false"/>
<item android:drawable="@color/light_grey_color" android:state_focused="false" android:state_pressed="true" android:state_selected="true"/>
<!-- Focused states -->
<item android:drawable="@color/xlight_grey" android:state_focused="true" android:state_pressed="true" android:state_selected="false"/>
<item android:drawable="@color/light_grey_color" android:state_focused="true" android:state_pressed="true" android:state_selected="true"/>
For me, I just used simple colors already defined in @color but that doesn't matter, you can use any picture from @drawable, or color from @color. Or in YOUR case, the 9 patch that you should make with the border on the bottom.
Then you need to define a style for tabs on your @style like this :
<style name="ThemeTabs" parent="@android:style/Theme.Holo.Light">
<item name="android:actionBarTabStyle">@style/ActionBarTabStyle.Myactionbar</item>
</style>
<style name="ActionBarTabStyle.Myactionbar" parent="@android:style/Widget.Holo.Light.ActionBar.TabView">
<item name="android:background">@drawable/tabs_backgrounds</item>
</style>
The on your manifest.xml, you just add this theme for the activity that has the tabs. Something like this :
<activity
....
android:theme="@style/ThemeTabs"
.... >
</activity>
I find that this is the best way to customize the tabs, you can choose a color or a drawable for each state (focused, clicked ...)
I hope this could help :)