quick_info_class

specifies the CSS class that will be applied to the pop-up edit form

start	Date	the date when a task is scheduled to begin
end	Date	the date when a task is scheduled to be completed
task	Task	the task object

gantt.templates.quick_info_class = function(start, end, task){ 
    return task.type == gantt.config.types.milestone ? "milestone_popup" : "";
};

Details

This template is defined in the Quick Info extension, so you need to activate the quick_info plugin.