This is how I changed the background of the ADD TO CART button
a. You want to make changes in the style.css file by default located here
Storefront\view\default\stylesheet\style.css
b. For both views product list and product page, change the value of the property background in css classes productcart and productpagecart.
b.1. Product list view (style shown in category view, featured, latest products, specials & search results)
style.css around line 1015
.thumbnail a.productcart {
background: #000; /* background color */
color: #fff; /*Font color */
float: right;
padding: 8px 27px 8px 8px;
font-size: 13px;
text-transform: uppercase;
position:relative;
}
The hover is a little after in line 1042:
.thumbnail a.productcart:hover {
background: #0e5f86; /* background color */
}
b.2. Product page view, in style.css line 1576
.productpagecart a {
background: #000; /* background color */
color: #fff; /* Font color */
border-radius: 5px;
border-width: 1px 0 0;
padding: 15px 15px 15px 60px;
font-size: 18px;
position:relative;
}
And the hover located in style.css line 1606
.productpagecart li a:hover {
background-color: #0e5f86; /* background color */
}