Hi,

Try to add the following code snippet to the Custom CSS area in Theme Settings to do this

.product-type-simple form.cart, 
.product-type-variable .variations_button { 
	display: -webkit-box; 
	display: -ms-flexbox; 
	display: flex; 
	-webkit-box-pack: row; 
	-ms-flex-pack: row; 
	justify-content: row; 
}

.product-type-simple form.cart .button, 
.product-type-variable .variations_button .button { 
	width: 100%; 
	font-size: 20px;
}

Regards