The above CSS is working fine for both Chrome and Mozilla, but it's not working for Safari.
Mozilla:
@supports (-moz-appearance:none) {
  .additem .additem_btn i:before {
    font-size: 14px !important;
    margin-left: -2px !important;
  }
}
Safari:
_::-webkit-full-page-media,
_:future,
 :root .additem .additem_btn i:before {
  font-size: 14px !important;
  margin-left: -2px !important;
}
All browsers:
.additem .additem_btn i:before {
  font-size: 14px !important;
  margin-left: -1px !important;
}
Original design:
Issues are marked in red color:


 
    