Why cant I change the android:windowBackground?

So I'm trying to use a custom theme for my android app, the idea is to change the color (or even add an image) while the engine is loading (before unity logo) since some lower-end devices are taking up to 10 seconds to show the splash and a 10 seconds blackscreen just makes it look like the app is broken.

So far my android manifest has:

 android:theme="@style/CustomTheme"

I have another file with the styles:

 <resources>
     <style name="CustomTheme" parent="@android:style/Theme.Material.NoActionBar.Fullscreen">
         <item name="android:windowBackground">@drawable/editor_bg</item>
     </style>
 </resources>

Whenever I change the style's parent, say to Theme.Translucent.NoTitleBar, I see the change, my app removes the black screen so I know the style It's been changed/used.

My problem is that @drawable/editor_bg doesn't seem to work at all, I even tried to just load a color instead of an image with @color/white, defined in another file.

Not sure if it is a problem with unity itself (using version 5.3.1 personal) or android. Any help is appreciated.