I am not sure which border you refer to. This is why I suggest taking a look at this article
to find out which element you need to customize. After that you can follow the steps described here
in order to customize the corresponding UI element.
Another approach that may also work in your scenario is to create a custom Style targeting the RadTreeViewItem control and setting it through the ItemContainerStyle property of the RadTreeView. You can find more information on that matter in our documentation
However, if the RadTreeViewItem does not expose public property controlling the border that you refer to, you will need to use the first approach.
Pavel R. Pavlov
Check out Telerik Analytics, the service which allows developers to discover app usage patterns, analyze user data, log exceptions, solve problems and profile application performance at run time. Watch the videos
and start improving your app based on facts, not hunches.