How to put a CardView attribute in a style?

Set parent attribute to CardView.
You don’t even have to add xmlns:card_view="http://schemas.android.com/apk/res-auto".

Working snippet of code:

<style name="CardViewStyle" parent="CardView">
     <item name="cardCornerRadius">4dp</item>
     <item name="cardElevation">4dp</item>
</style>