You can use the onBeforeTaskRender event handler to customize the task boxes.
This event will let you set custom color easily.
{
onBeforeTaskRender: (args) => {
args.data.box.backColor = "#cc0000";
},
// ...
}
For more complex styling (like using a striped background for the progress bar), you can add a custom CSS class to the task box and style the elements using CSS.
{
onBeforeTaskRender: (args) => {
args.data.box.cssClass = "my-task";
},
// ...
}