LibreTube/app/src/main/res/drawable/player_switch_thumb.xml

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">
     <item android:state_checked="false"
        android:drawable="@drawable/ic_pause_circle"/>

    <item android:state_checked="true"
        android:drawable="@drawable/ic_play_circle"/>

</selector>