There are a couple appearance options (Vertigo and Sidebar) which don't have the button outlines, may be closer to what you are after. They also don't have the

There are a couple appearance options (Vertigo and Sidebar) which don't have the button outlines, may be closer to what you are after. They also don't have the dotted branch lines that some tree controls have. There is also a "No Decoration" option where you can add your own style rules in the Advanced section.