It looks like the only way is to clear the style for _todaybox in the CSS (to hide the default today indicator) and add a custom class using BeforeCellRender for today.
protected void DayPilotNavigator1_BeforeCellRender(object sender, DayPilot.Web.Ui.Events.Navigator.BeforeCellRenderEventArgs e)
{
// Highlight Today
if (e.Start.ToShortDateString() == GetLocalTime(DateTime.Now).ToShortDateString()) {
e.CssClass = "today";
}
}
The custom class is not tested, you may need to tweak it a bit:
.today {
box-sizing: border-box; /* make sure the border doesn't change the dimensions */
border: 1px solid red;
}