1. Decompile framework-res.apk
2. Navigate to res/values and open styles.xml
3. Locate <style name="Theme">
4. Replace
<item name="colorBackground">@color/background_dark</item>
with
<item name="colorBackground">@color/transparent</item>
5. Replace
<item name="windowBackground">@drawable/screen_background_dark</item>
with
<item name="windowBackground">@drawable/background</item>
6. Locate <style name="Theme.Black" parent="@style/Theme">
7. Replace
<item name="colorBackground">@color/black</item>
with
<item name="colorBackground">@color/transparent</item>
8. Replace
<item name="windowBackground">@color/black</item>
with
<item name="windowBackground">@drawable/background</item>
9. Create a 320X480 png to your liking, name it background.png and place it in drawable-mdpi folder
10. Recompile framework-res.apk