To reproduce:
- Set ShowRootLines to true
- Apply one of the following themes: Aqua, Office2007, TelerikMetro, VS2012

Workaround:
- Open the theme in visual style builder
- Expand the RadTreeView in the ControlStructureWindow
- Select TreeNodeExpandItem and expand it in the Elements window
- Navigate to the PossitionOffset property and set its width to 1