can we change the expander or collapse icon

You can change the icons using CSS. This is how the icons are defined in the default CSS theme:

.gantt_default_tree_image_expand {
    background-image: url('data:image/svg+xml;base64,PHN2ZyB4bWxucz0naHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmcnPjxwYXRoIGQ9J00gMS41IDAuNSBMIDYuNSA1IEwgMS41IDkuNScgc3R5bGU9J2ZpbGw6bm9uZTtzdHJva2U6Izk5OTk5OTtzdHJva2Utd2lkdGg6MjtzdHJva2UtbGluZWpvaW46cm91bmQ7c3Ryb2tlLWxpbmVjYXA6YnV0dCcgLz48L3N2Zz4=');
}

.gantt_default_tree_image_collapse {
    background-image: url('data:image/svg+xml;base64,PHN2ZyB3aWR0aD0nMTAnIGhlaWdodD0nMTAnIHhtbG5zPSdodHRwOi8vd3d3LnczLm9yZy8yMDAwL3N2Zyc+PHBhdGggZD0nTSAwLjUgMS41IEwgNSA2LjUgTCA5LjUgMS41JyBzdHlsZT0nZmlsbDpub25lO3N0cm9rZTojOTk5OTk5O3N0cm9rZS13aWR0aDoyO3N0cm9rZS1saW5lam9pbjpyb3VuZDtzdHJva2UtbGluZWNhcDpidXR0JyAvPjwvc3ZnPg==');
}

Thanks, will try this.