As you can see for RadGrid's NestedViewTemplate which is a similar feature, there is no predefined CssClass and it is styled by setting the CssClass of the container control inside - which may vary from a <div> to an asp:Panel. More built-in css classes mean more mark-up to be rendered even when it is not needed, that is why this class was not added.
However, since you mentioned you use jQuery, an easy way to add a custom CSS would be to pick all <tr> tags in the treelist that contain an rtlA or rtlR CSS class and do not have an id (this will select all treelist detail items) and append the custom class name:
I hope this helps.
the Telerik team
Registration for Q1 2011 What’s New Webinar Week is now open. Mark your calendar for the week starting March 21st and book your seat
for a walk through all the exciting stuff we ship with the new release!