DayPilot.Gantt - Change/modify links on render?

At this moment, there is no special onBeforeLinkRender event.

However, you can set the properties directly in the data source. You can customize the look using properties like color, style, and cssClass (see the API docs):

const links = [
  {
     from: 1,
     to: 2,
     color: "#00cc00",
     cssClass: "my-link"
  }
];
dp.update({links});