On our site we are using the Visualization module and the Highcharts library. We have a column chart. I need the fill to be transparent. When I look at the source, I see this:

<rect class=" highcharts-background" x="0" y="0" width="512" height="400" strokeWidth="0" fill="#FFFFFF">

I don't even know how to find out which theme is being used. Where can this option be set up? Thanks