Hi,
In the file templates/dykkerhulen/css/style.css of your template, you have that CSS which handle the HikaShop add to cart buttons:
.box-background-content.sidebar-box > div .card-body #login-form .button, .rs-form-content .formControls .formBody .rsform-submit-button, .rs-form-content #signup.rsform-submit-button, .hikabtn.hikacart {background: #658f28; border: 0px; border-radius: 0px; color: #ffffff; font-size: 15px; font-weight: 700; height: 45px; line-height: 45px; padding: 0 45px; text-transform: uppercase; margin-top:15px;}
.box-background-content.sidebar-box > div .card-body #login-form .button:hover, .rs-form-content .formControls .formBody .rsform-submit-button:hover, .rs-form-content #signup.rsform-submit-button:hover, .hikabtn.hikacart:hover{background: rgba(0, 0, 0, 0); box-shadow: 0 0 0 2px #658f28 inset; color: #658f28;}
As you can see it has the classes .hikabtn.hikacart twice there to handle the add to cart buttons, but it's missing the classes .hikabtn.hikawishlist to handle the add to wishlist buttons. If you add these classes in there, the add to wishlist buttons will display like the add to cart buttons.