Why does the background in design suddenly change to gray when using CardView?

Question

Java

Why does the background in design suddenly change to gray when using CardView? I change CardView to any other layout and the color becomes white. How to fix it? (unpleasant to work)
Here is the card code

<android.support.v7.widget.CardView
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:card_view="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:layout_margin="7dp"
    card_view:cardCornerRadius="5dp">

</android.support.v7.widget.CardView>