Currently no way to theme the preview box.

Submitted via email but resubmitting here to open the conversation. Attached is a patch which adds class names to the pieces of the preview box as well as a css file reference for the module as a whole. Right now, the module has no way to layer any style changes so admins are stuck with the out-of-the-box look. Also, the addition of a css file for the module allows admins the ability to universally adopt a look for the preview box and then tweak it with specific overrides via individual themes. Leaving out the module CSS path means the default look for upload_preview is still whatever comes out of the box instead of what the site administrator designs.