It looks like you're missing transparency on the track drawable, so the thumb isn't visible when the style is applied. Try adding this to your @drawable/track.xml:
<?xml version="1.0" encoding="utf-8"?>
<shape
xmlns:android="http://schemas.android.com/apk/res/android"
android:shape="rectangle" >
<!-- add transparency to ensure thumb visibility -->
<solid
android:color="#00000000" />
<stroke
android:width="1dp"
android:color="@android:color/black" />
<size
android:width="2dp" />
</shape>