app/src/main/res/drawable/thumb.xml

<?xml version="1.0" encoding="utf-8"?>
<selector
    xmlns:android="http://schemas.android.com/apk/res/android">
    <item>
        <shape
            android:shape="oval">
            <solid android:color="#fff"/>
            <stroke
                android:color="#808080"
                android:width="0.6dp"/>
            <size
                android:height="32dp"
                android:width="32dp"/>
        </shape>
    </item>
</selector>