fillPatternStyle and fillPatternColor ignored on cells

Setting fillPatternStyle on a cell produces patternType="solid" in the output instead of the specified pattern.

node -e '
  var w = require("write-excel-file/node")
  w([[{value:"x", type:String, backgroundColor:"#ff0000", fillPatternStyle:"darkGray", fillPatternColor:"#0000ff"}]],
    {filePath:"/tmp/a.xlsx"}
  ).then(function() {
    require("child_process").execSync("unzip -o /tmp/a.xlsx -d /tmp/a >/dev/null 2>&1")
    console.log(require("fs").readFileSync("/tmp/a/xl/styles.xml", "utf8").match(/patternType="[^"]+"/g))
  })'

Actual: patternType="none", patternType="gray125", patternType="solid"

Expected: patternType="none", patternType="gray125", patternType="darkGray"