Exemplo de LESS

style.css

body {
  padding-top: 60px;
}
.hero-unit img {
  margin: -50px -30px 0px 0px;
  float: right;
}
.btn.tiny {
  padding: 2px 4px;
  font-size: 11px;
  -webkit-border-radius: 3px;
  -moz-border-radius: 3px;
  border-radius: 3px;
}
.modal form.form-stacked {
  margin: 0px;
  padding: 0px;
}
.modal form.form-stacked div.field {
  display: inline-block;
  vertical-align: top;
  margin: 0px;
  padding: 0px;
}
.modal form.form-stacked div.field.error {
  background: white;
}
.modal form.form-stacked div.error input {
  border-color: #c87872;
  -webkit-box-shadow: 0 0 3px rgba(171, 41, 32, 0.5);
  -moz-box-shadow: 0 0 3px rgba(171, 41, 32, 0.5);
  box-shadow: 0 0 3px rgba(171, 41, 32, 0.5);
}
.modal form.form-stacked div.error span.help-inline {
  color: #9D261D;
}

style.less

.box-shadow (@x, @y, @blur, @color) {
  -webkit-box-shadow: @x @y @blur @color;
  -moz-box-shadow: @x @y @blur @color;
  box-shadow: @x @y @blur @color;
}

.border-radius(@radius) {
  -webkit-border-radius: @radius;
  -moz-border-radius: @radius;
  border-radius: @radius;
}

body {
  padding-top: 60px;
}

.hero-unit {
  img {
    margin: -50px -30px 0px 0px;
    float: right;
  }
}

.btn.tiny {
  padding: 2px 4px;
  font-size: 11px;
  .border-radius(3px);
}

.modal {
  form.form-stacked {
    margin: 0px;
    padding: 0px;
    
    div.field {
      display: inline-block;
      vertical-align: top;
      margin: 0px;
      padding: 0px;

      &.error {
        background: white;
      }
    }

    div.error {
      input {
        border-color: #c87872;
        .box-shadow(0, 0, 3px, rgba(171, 41, 32, 0.5));
      }
      span.help-inline {
        color: #9D261D;
      }
    }
    
  }
}