gistfile1.txt

/* River lines */

.river { line-color: #acf; }

/* I am guessing as to the meaning of the CLASS column. 
 * This style appears mostly correct */

.river[zoom=7][CLASS=1] { line-width: 0.1; }
.river[zoom=8][CLASS=1] { line-width: 0.2; }
.river[zoom=9][CLASS=1] { line-width: 0.4; }
.river[zoom=10][CLASS=1] { line-width: 0.8; }
.river[zoom=11][CLASS=1] { line-width: 1.6; }
.river[zoom=12][CLASS=1] { line-width: 2.4; }

.river[zoom=8][CLASS=2] { line-width: 0.1; }
.river[zoom=9][CLASS=2] { line-width: 0.2; }
.river[zoom=10][CLASS=2] { line-width: 0.4; }
.river[zoom=11][CLASS=2] { line-width: 0.8; }
.river[zoom=12][CLASS=2] { line-width: 1.6; }

.river[zoom=9][CLASS=3] { line-width: 0.1; }
.river[zoom=10][CLASS=3] { line-width: 0.2; }
.river[zoom=11][CLASS=3] { line-width: 0.4; }
.river[zoom=12][CLASS=3] { line-width: 0.8; }