In order to customize the card radius, you need to follow the following steps:-
Go to the file "defaultTheme.js" placed at the following path:-
In this file, an object named "defaultTheme" is defined which contains properties related to the structural elements of the template. In this object, there is a property name "overrides". "overrides" is an object, it contains the "MuiCard" property which again is an object. In the "MuiCard" object, you will find a property named "borderRadius". This is the property responsible for the card radius.
Set the value of "borderRadius" according to the radius you want to give to the card.