As usual you have full freedom to select what grid lines you like to show (and what colors they should have). There are three different types of grid lines you may adjust. The radius minor and major grid lines and the angle grid lines.
You select what grid lines to show with a call to PolarAxis::ShowGrid() The two example below shows a logarithmic plot with either just major grid lines or both minor and major grid lines.
The colors of the grid lines are specified with a call to PolarAxis::SetGridColor()
For the angle grid lines it is possible to specify the angle division between each grid line with the method PolarAxis::SetAngleStep() You specify the step distance in degrees. By default the step size is 15 degrees.