@mixin xs() {
 @media only screen and (max-width: $screen-xs-max) {
   @content;
 }
}

@mixin sm() {
 @media only screen and (min-width: $screen-sm-min) and (max-width: $screen-sm-max) {
   @content;
 }
}

@mixin md() {
 @media only screen and (min-width: $screen-md-min) and (max-width: $screen-md-max) {
   @content;
 }
}

@mixin lg() {
 @media only screen and (min-width: $screen-lg-min) {
   @content;
 }
}