function scale(v) {
    v = (Math.min(v, 255) * 257).toString(16);
    return lib.f.zpad(v, 4);
  }
Convert a CSS rgb(ddd,ddd,ddd) color value into an X11 color value.

Other CSS color values are ignored to ensure sanitary data handling.

Each 'ddd' component is a one byte value specified in decimal.

@param {string} value The CSS color value to convert.
@return {?string} The X11 color value or null if the value could not be
    converted.