Put your own mixins in

mixins-example.scss

@mixin size($size) {
  @if length($size) == 2 {
    $width  : nth($size,1);
    $height : nth($size,2);
    
    @if unitless($width) {
      $width: $width + 0px;
    }
    
    @if unitless($height) {
      $height: $height + 0px;
    }

    height: $height;
    width: $width;
  }
  
  @if length($size) == 1 {
    @if unitless($size) {
      $size: $size + 0px;
    }

    height: $size;
    width: $size;
  }
}

.welcome {
  @include size(480 120);
  color: $color_prim_03;
  font-size: $font_size*4;
  letter-spacing: 10px;
  line-height: $font_size*14;
  position: absolute;
  top: 50%;
  left: 50%;
  margin-top: -50px;
  margin-left: -240px;
  text-align: center;
  border: 2px solid $color_prim_03;
  &:hover {
    margin-top: -60px;
    margin-left: -250px;
    @include box-shadow(10px 10px 0 $color_prim_03);
  }
}