How can I make my activity have a transparent background?

How can I make my activity have a transparent background?

I don't want to use
android:theme="@android:style/Theme.Translucent.NoTitleBar" because the
progressbar come out in black in an old outdated style. so I took what's
inside this theme and put ito my own style with Holo Light as parent.
<style name="noTitleTransparentBackground" parent="android:Theme.Holo.Light">
<item name="android:windowNoTitle">true</item>
<item name="android:windowContentOverlay">@null</item>
</style>
But this made the background not transparent anymore.
What am i doing wrong?