In order to customize the look (i.e. style and template) of a control from Gantt Chart Library for WPF or Project Management Library for WPF, you can start by using the default control style (or template) value which you can obtain, in clear XAML, from the Generic.xaml file from our Gantt Chart Control Styles sample archive, and apply your own changes to it, for your control instances.
You can also review the Control Templating sample applications for the control types that you want to customize.