Use +/- to Expand/Contract Icons in AdvTreeView

I am using an AdvTreeView and I would like to change the icons used to show whether a node can  expand/contract;

The icons seem stuck at >/v and I would like the default +/- to show a node can be expanded.

The default expand/collapse icons in TAdvTreeView are +/- 

The icons can be changed with the properties: AdvTreeView.NodesAppearance.ExpandNodeIcon, AdvTreeView.NodesAppearance.CollapseNodeIcon