Set activity screen orientation using styles

I'm trying to set activity screen orientation using a styles:

Here is the styles.xml:

...<style name="Some.Activity" parent="android:Theme"><item name="android:screenOrientation">portrait</item></style>...

and here is activity declaration in AndroidManifest.xml:

<activity android:name="SomeActivity" android:theme="@style/Some.Activity"/>

and it doesn't work.
I know that I can just set

<activity android:name="SomeActivity" android:theme="@style/Some.Activity"    android:screenOrientation="..."/>

in the manifest, but its important to me to do that using a theme.
Ideas?