score:1

Accepted answer

Set a 'gridLineWidth' and 'gridLineColor' in the x-axis.

xAxis: {
                categories: ['Label', 'Label', 'Label', 'Label', 'Label', 'Label', 'Label', 'Label', 'Label', 'Label', 'Label'],
                title: {
                    text: null
                },
                tickLength: 500,
                lineColor: 'transparent',
                labels: {
                    style: {
                        color: '#ffffff'
                    }
                },
                gridLineWidth:'1px',
                gridLineColor:'#FFFFFF'

            },

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