Create /res/drawable/button.xml with the following content :
<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android"
android:shape="rectangle" android:padding="10dp">
<!-- you can use any color you want I used here gray color-->
<solid android:color="#90EE90"/>
<corners
android:bottomRightRadius="3dp"
android:bottomLeftRadius="3dp"
android:topLeftRadius="3dp"
android:topRightRadius="3dp"/>
</shape>
And then you can use the following :
<Button
android:id="@+id/button_save_prefs"
android:text="@string/save"
android:background="@drawable/button"/>