I have created the action bar by
ActionBar actionbar = getActionBar()
The background of the action bar is changed by
acti
I'll suggest you use ActionBarSherlock. There is one sample available in the library named "Style ActionBar". (this is only way you can change ActionBar tabs underline color)
if you have customized ActionBar then You have to add this style in ActionBar Style
or here is way how to Do this

create style like below (here i have used ActionBarShareLock if you don't want to use then use android-support-v4.jar for support all Android OS Version)