quaggaJS/example/sass/phone/_core.scss

@include phone {  
  #container{
    width: 300px;
    margin: 10px auto;
    @include box-shadow(none);
    
    form.voucher-form{
      input{
        &.voucher-code{
          width: 180px;
        }
      }
    }
  }
}