tree-widget-theme

tree-widget-theme is a variable defined in `tree-widget.el'.
Its value is

nil

Documentation:
Name of the theme in which to look for images.
This is a sub directory of the themes directory specified by the
`tree-widget-themes-directory' option.
The default theme is "default". When an image is not found in a
theme, it is searched in its parent theme.

A complete theme must at least contain images with these file names
with a supported extension (see also `tree-widget-image-formats'):

"guide"
A vertical guide line.
"no-guide"
An invisible vertical guide line.
"end-guide"
End of a vertical guide line.
"handle"
Horizontal guide line that joins the vertical guide line to an icon.
"no-handle"
An invisible handle.

Plus images whose name is given by the :glyph-name property of the
icon widgets used to draw the tree. By default these images are used:

"open"
Icon associated to an expanded tree.
"close"
Icon associated to a collapsed tree.
"empty"
Icon associated to an expanded tree with no child.
"leaf"
Icon associated to a leaf node.

You can customize this variable.