UnrealEngineUWP/Engine/Build/Android/Java/res/values-port/styles.xml

<?xml version="1.0" encoding="utf-8"?>
<resources>

    <style name="UE4BaseTheme" parent = "@android:style/Theme.Black.NoTitleBar.Fullscreen" />

    <style name="UE4SplashTheme" parent = "UE4BaseTheme">
        <item name="android:windowBackground">@drawable/splashscreen_portrait</item>
        <item name="android:windowNoTitle">true</item>
        <item name="android:windowFullscreen">true</item>
    </style>

</resources>