The icons for this widget template are available in Sitefinity 8.0, here
is a screenshot.
Those icons should be loaded so first check web.config if there is a rule set to disable embedded skins for RadTreeView control
In Sitefinity 8.0 a specific is that Telerik web controls don`t use skin Sitefinity anymore which was used for almost all controls. The skin Sitefinity has been removed from Telerik.Web.UI.Skins.dll and now the skin file used is Skin="Default".
In case the navigation template for the vertical tree with sub levels have a rule for using Skin="Sitefinity" restore this template to Default
and test again for the icons.
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Sitefinity CMS Ideas&Feedback Portal
and vote to affect the priority of the items