Changing HighCharts background color?

That's you want `

chart: {
            polar: true,
            type: 'line',
            backgroundColor:'rgba(255, 255, 255, 0.1)'
        }

`

Take a look at the Highcharts API here: https://api.highcharts.com/highcharts/chart.backgroundColor and you will see it's a property of the chart object that can take a solid color:

{
  chart: {
     backgroundColor: '#FCFFC5',
     polar: true,
     type: 'line'
  }
}

Or Gradiant:

{
  chart: {
    backgroundColor: {
       linearGradient: [0, 0, 500, 500],
       stops: [
         [0, 'rgb(255, 255, 255)'],
         [1, 'rgb(200, 200, 255)']
       ]
     },
     polar: true,
     type: 'line'
  }
}

There is a backgroundColor tag you can use: (http://jsfiddle.net/aeP4E/)

chart: {
      polar: true,
      type: 'line',
      backgroundColor:'blue' 
},