Hi Xavier,
thank you for your post!
I've fixed it up by changing size of the picture - 19 px instead of 35 px:
hikashop_next_product {
background-color: rgba(0, 0, 0, 0) !important;
background-image: url("../images/icons/next.png");
float: right;
height: 19px;
width: 19px
I think this is not to do with CSS customization but smth with Configuration --> System --> -->Main --> Images --> "Force images size". Such presumption is based on experience with oversized pictures for categories right after updating.