zumo_2.0_android/app/src/main/res/drawable/black_button_pressed.xml

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">
  <item>
    <shape android:shape="rectangle">
      <stroke android:width="1dp" android:color="@color/C000000"/>
      <solid android:color="@color/C66000000"/>
      <corners android:radius="2dp"/>
    </shape>
  </item>
</selector>