Hint:

The update to the last version of the library changed the color of the Gantt Lines (Axis-line and tick), if needed to change for the color of the previous version you can add the follow CSS in the theme of your application:

.highcharts-axis-line {

      stroke: rgb(204, 214, 235);

}

.highcharts-tick {

      stroke: rgb(204, 214, 235);

}