Layer CSS trick

gistfile1.txt

<script type="cartocss/html" id="pm25_na">
   /*Country Styles*/

#pm25_na{
  polygon-fill: #FFFFB2;
  polygon-opacity: 0.8;
  line-color: #FFF;
  line-width: 1;
  line-opacity: 1;
}

#pm25_na [ {0} <= 154] {
   polygon-fill: #000000;
}
#pm25_na [ {0} <= 140] {
   polygon-fill: #1A0800;
}
#pm25_na [ {0} <= 125] {
   polygon-fill: #241105;
}
#pm25_na [ {0} <= 110] {
   polygon-fill: #301703;
}
#pm25_na [ {0} <= 90] {
   polygon-fill: #3F1C05;
}
#pm25_na [ {0} <= 69] {
   polygon-fill: #4D1E04;
}
#pm25_na [ {0} <= 55] {
   polygon-fill: #631E04;
}

#pm25_na [ {0} <= 49.92] {
   polygon-fill: #791406;
}
#pm25_na [ {0} <= 32.54] {
   polygon-fill: #970707;
}
#pm25_na [ {0} <= 25.17] {
   polygon-fill: #c30000;
}
#pm25_na [ {0} <= 15.17] {
   polygon-fill: #f50000;
}
#pm25_na [ {0} <= 10.85] {
   polygon-fill: #FF3200;
}
#pm25_na [ {0} <= 9.15] {
   polygon-fill: #FF5a00;
}
#pm25_na [ {0} <= 8.15] {
   polygon-fill: #FF8200;
}
#pm25_na [ {0} <= 6.06] {
   polygon-fill: #FFaa00;
}
#pm25_na [ {0} <= 4.62] {
   polygon-fill: #ffd72d;
}
#pm25_na [ {0} <= 2.98] {
   polygon-fill: #fff74b;
}



    </script>