[Solved]-How to customize tooltip styling in for Scatter plot in Highchart?-Highcharts

You need to define tooltip options on the main level:

Highcharts.chart('container', {
    ...,
    tooltip: {
        headerFormat: '...',
        pointFormat: '...',
        backgroundColor: 'black',
    }
});

Live demo: http://jsfiddle.net/BlackLabel/6gxcanhs/

API Reference: https://api.highcharts.com/highcharts/tooltip