CSS3 hover transition property

Question

@mixin secondary-btn-color {
  border-color: $grey_light_dark;
  background: #ffffff;
  color: $grey_dark;
  &:hover {
    border-color: darken($grey_light_dark, 8%);
    background: $grey_lightest;
    color: darken($grey_light_dark, 18%);
  }
  .fa {
    color: $grey_dark;
  }
}