[Solved]-How do I change the style of the frame lines in a radar(polar) chart created with highcharts?-Highcharts

1. Use plot lines for y-axis:

   yAxis: {
       ...
       plotLines: [{
           color: 'red',
           value: 100,
           zIndex: 2
       }]
   }
   

2. Use gridLineColor for x-axis:

   xAxis: {
       ...,
       gridLineColor: 'blue'
   }
   

3. Use gridLineColor for y-axis:

   yAxis: {
       gridLineColor: 'green',
       ...
   }
   

Live demo: http://jsfiddle.net/BlackLabel/fm45ab3k/

API Reference: https://api.highcharts.com/highcharts/yAxis.plotLines