My fault... "node icons are of course the NODE icons, not the JOINT" icons
EDIT: is there any way to change those icons? In GXT2 this was no problem. Now those icons are managed by the TreeAppearance - but I can't find any way to set my own TreeAppearance (only GridAppearance)..
Yes, we do need to expose a way to set the icons (and other appearance details) for the Tree in the TreeGrid on an individual basis, but the theming setup still works - you can still configure the entire application's theme to have a specific appearance impl.
The interface com.sencha.gxt.widget.core.client.tree.Tree.TreeAppearance gets its implementation from these lines in com/sencha/gxt/theme/blue/Blue.gwt.xml:
We have added support for joint icons with TreeStyle. So you can change icons the same way you change the node icons. We also added a constructor that allows you to specify a TreeAppearance. Change will go out in next release later this week.