Looking at the details in stylesheet_colors.css reveals that the
Add Selected to Cart button on product listing pages and the
Add to Cart button on product info pages were apparently intended to have the same coloring, but the cascading overruled this. Making the selectors more specific fixes this.
Code:
/* Product Listing Pages */
.btn.button_add_selected {background:#00C851;color:#FFF;}
.btn.button_add_selected:hover {background:#007E33;color:#FFF;}
/* Product Info Pages */
#addToCart-card-header, .btn.button_in_cart {background:#00C851;color:#FFF;}
.btn.button_in_cart:hover {background:#007E33;color:#FFF;}
#addToCart-card {border-color:#00C851;}