[Solved]-How to change the background color the chart area in high charts?-Highcharts

If you want to adjust the color for this series, you can use https://api.highcharts.com/highcharts/plotOptions.series.color. So you would just set your series like:

series: [{
    showInLegend: false,
    data: [
        [0, 29.9],
        [1, 71.5],
        [3, 106.4],
        [4, 300.4],
        [5, 400.4],
        [6, 20.4],
        [7, 40.4],
        [8, 120.4],
        [9, 50.4],
        [10, 230.4],
        [11, 110.4],
        [12, 500.4],
    ],
    color: '#ff0000'
}]

See a working demo at https://jsfiddle.net/gh/get/library/pure/highcharts/highcharts/tree/master/samples/highcharts/plotoptions/series-color-specific/ from their documentation or https://jsfiddle.net/pkcrt5q2/ for your specific example.

Additionally, you can set this as a global option as described at https://api.highcharts.com/highcharts/colors.

Highcharts.setOptions({
    colors: ['#ff0000']
});

Working demo: https://jsfiddle.net/gh/get/library/pure/highcharts/highcharts/tree/master/samples/highcharts/chart/colors/