breakpoints.css

// phone only
@media (max-width: 599px) {
}

// tablet portrait up
@media (min-width: 600px) {
}

// tablet landscape up
@media (min-width: 900px) {
}

// desktop up
@media (min-width: 1200px) {
}

// big desktop up
@media (min-width: 1800px) {
}

breakpoints.sass

@mixin for-phone-only {
  @media (max-width: 599px) { @content; }
}
@mixin for-tablet-portrait-up {
  @media (min-width: 600px) { @content; }
}
@mixin for-tablet-landscape-up {
  @media (min-width: 900px) { @content; }
}
@mixin for-desktop-up {
  @media (min-width: 1200px) { @content; }
}
@mixin for-big-desktop-up {
  @media (min-width: 1800px) { @content; }
}

// usage
.my-box {
  padding: 10px;
  
  @include for-desktop-up {
    padding: 20px;
  }
}