This feature isn't quite supported yet. There was some SDK level work done to support this, but there is currently no way for the creative to tell the SDK that it wants to be transparent. The team needs to do more testing and run some experiments to determine that transparent interstitials will have a positive impact and are an overall better user experience. I don't have a date for when this will be completed though.
To prepare your app to support transparent interstitials later, you'll need to add Theme.Translucent to your AdActivity declaration in your Android manifest:
<activity android:name="com.google.android.gms.ads.AdActivity"
android:theme="@android:style/Theme.Translucent"
android:configChanges="keyboard|keyboardHidden|orientation|screenLayout|uiMode|screenSize|smallestScreenSize"/>
That'll tee your app up for support later down the road.