Mixin for adding breakpoints

breakpointMixin.scss

@mixin bp-large {
  @media only screen and (max-width: 60em) {
    @content;
  }
}

@mixin bp-medium {
  @media only screen and (max-width: 40em) {
    @content;
  }
}

@mixin bp-small {
  @media only screen and (max-width: 30em) {
    @content;
  }
}

usage

.sidebar {
  width: 60%;
  float: left;
  margin: 0 2% 0 0;
  @include bp-small {
    width: 100%;
    float: none;
    margin: 0;
  }
}