Hello,
Go to Theme Settings -> Styles and Colors -> Buttons.
https://xtemos.com/docs-topic/advanced-button-styles/
Alternatively please add this code to the Theme Settings > Custom CSS > Global for add to cart button.
.single_add_to_cart_button, .wd-single-add-cart .single_add_to_cart_button{
background-color: white;
color: black;
border: 2px solid #4CAF50; /* Green */
box-shadow: 0 8px 16px 0 rgba(0,0,0,0.2), 0 6px 20px 0 rgba(0,0,0,0.19);
}
Change the code as per your need.
Best Regards.