Hello,
1/
/* Add to cart button on archives *\/
.content-product .add_to_cart_button {}
/* Add to cart button on single product page & in quick view */
.single_add_to_cart_button.button {}
2/ You may add custom CSS in few different places
2.1/ Customizer -> Theme Custom CSS -> Global CSS (saved in database)
2.2/ Customizer -> Additional CSS (saved in database)
2.3/ xstore-child/style.css (saved in file)
Regards