legend.itemStyle doesn't seem to be accepting a Highcharts.CSSObject. When trying to apply:

legend: { itemStyle: { color: "#5f5e5e", fontFamily: "Roboto", fontSize: "12px" } }, 

I get this error:
value ({'color': '#5f5e5e', 'fontFamily': 'Roboto', 'fontSize': '12px'}) was not coerced to a string