When nil use the directory where the tree-widget library is located.
When it is a relative name, search in all occurrences of that sub
directory in the path specified by `tree-widget-themes-load-path'.
When nil use the directory where the tree-widget library is located.
When it is a relative name, search in all occurrences of that sub
directory in the path specified by `tree-widget-themes-load-path'.