discourse/app/assets/stylesheets/common/base/upload.scss

.add-upload .fa-plus {
  font-size: 10px;
  position: relative;
  left: -5px;
  bottom: -5px;
  text-shadow:
   -1px -1px 0 scale-color($tertiary, $lightness: -10%),
    1px 1px 0 scale-color($tertiary, $lightness: -10%),
    1px -1px 0 scale-color($tertiary, $lightness: -10%),
    -1px 1px 0 scale-color($tertiary, $lightness: -10%);
}