Well, you are almost there, you are getting the Explorer theme style. Just one wrong assumption, the theme for a treeview does not prescribe a font. Quite visible in the .NET TreeView class for example, you can arbitrarily assign its Font property and you'll get the requested font. And logically obvious, you didn't specify a font size anywhere so there's no reasonable way for the theming api to guess at a properly sized font. Right now you are just getting the default font that's selected in the device context.
You have to select the font you want to use.