This is a TileMill style for http://donutholes.ch base layer

style.mss

// Layers used to create the basemap:
// vector: http://mapbox-geodata.s3.amazonaws.com/natural-earth-1.4.0/physical/10m-bathymetry.zip // #bathymetry layer
//         http://mapbox-geodata.s3.amazonaws.com/natural-earth-1.4.0/physical/10m-ocean.zip // #ocean layer
//         http://mapbox-geodata.s3.amazonaws.com/natural-earth-1.4.0/cultural/10m-admin-0-countries.zip // #countries layer
//         http://mapbox-geodata.s3.amazonaws.com/natural-earth-1.4.0/cultural/10m-admin-0-countries.zip // #countries layer
//         http://mapbox-geodata.s3.amazonaws.com/natural-earth-1.4.0/cultural/10m-urban-areas.zip // #urban areas layer
// raster: OB_LR/OB_LR.tif // #oceanshade (downloaded from http://www.naturalearthdata.com/downloads/10m-raster-data/10m-ocean-bottom/)
Map {
  background-color: #b8dee6;
}

@countryborder: #b8dee6;

#countries {
    polygon-fill:#0A202A;
    //polygon-pattern-opacity:10%;
    polygon-gamma:0.9;
	::glow-inner[zoom>0] {
		line-color:@countryborder;
  		line-width:0.8;
  		line-join:round;
  		line-opacity:0.4;
	}
  	::glow-innermiddle[zoom>1] {
  		line-color:@countryborder;
  		line-width:1;
  		line-join:round;
  		line-opacity:0.2;
	}
  	::glow-outermiddle[zoom>2] {
  		line-color:@countryborder;
  		line-width:1.2;
  		line-join:round;
  		line-opacity:0.1;
	}
  	::glow-outer[zoom>3] {
  		line-color:@countryborder;
  		line-width:1.2;
  		line-join:round;
  		line-opacity:0.05;
	}
}

#bathymetry {
  polygon-fill: hsl(120,15%,2%);
  polygon-comp-op: minus; 
}

#ocean {
  //polygon-fill: #93b5cc;
  polygon-fill: hsl(199,35%,55%);
}

#oceanshade {
  raster-scaling: lanczos;
  raster-comp-op: multiply;
  raster-opacity: 1;
}

#urban {
  polygon-fill:#9F6;
  line-color:#9F6;
  [zoom<=3]{
    line-opacity:0.33;
    polygon-opacity:0.66;
  }
  [zoom=4]{
    line-opacity:0.25;
    polygon-opacity:0.50;
  }
  [zoom=5]{
    line-opacity:0.20;
    polygon-opacity:0.30;
  }
  [zoom=6]{
    line-opacity:0.1;
    polygon-opacity:0.15;
  }
  [zoom>6]{
    line-opacity:0;
    polygon-opacity:0;
  }

}