Just want to let you know that for me, css has been a quicker and more effective way to change style parameters for Highcharts. 

There were many properties that would not be accepted by callback even though they are documented on the Highcharts site. 

Here is some css that was effective for me:

.highcharts-background {
fill: #363636; 
stroke: #a4edba;
stroke-width: 2px;
}

.highcharts-title {
    fill: #fff !important;
    font-weight: bold;
    font-size: 3em;
}
 .highcharts-axis-title{

    fill: #bcbcbc !important;
}
.highcharts-axis-labels {
    fill: #bcbcbc !important;
}
.highcharts-xaxis-labels text{
    fill: #bcbcbc !important;
    color: #bcbcbc !important;
}
.highcharts-yaxis-labels text{
    fill: #bcbcbc !important;
    color: #bcbcbc !important;
}
.highcharts-label {
    fill: #fff !important;
    font-weight: light !important;
}
.highcharts-data-label text{
    fill: #fff !important;
    font-weight: light !important;
}
.highcharts-text-outline {
    stroke-width: 0px !important;
    font-weight: light !important;
}

Mark