tank-server/app/src/main/res/values-v21/styles.xml

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

    <!-- APP DEFAULT -->

    <style name="AppTheme" parent="AppTheme.Base">
        <item name="android:windowDrawsSystemBarBackgrounds">true</item>
        <item name="android:navigationBarColor">@color/colorPrimaryDark</item>
    </style>

</resources>