Please see my comment to the question: it depends on how exactly it should look, according to your requirements.
Perhaps the simplest way to achieve it would be CSS with background image:
https://developer.mozilla.org/en-US/docs/Web/CSS/background-image[
^].
Of course, you should apply a class with background image not to your tree view, but to enclosing block element, such as
div
or the page.
Be very careful! Using background images can easily make you main content poorly readable. You main goal should be clear readability, functionality, accessibility and good content, not decorations.
—SA