I have taken an existing theme, modified and renamed it - the mods are mostly in the color, e.g., I've resaved some of the graphics so they're blue instead of gray. I've also done a little tweaking to the css. I've been careful to edit the css file to include updated info for Theme Name, Description, Version, Author, Author URI, and Tags. From the Manage Themes page in the Dashboard, I can see the screenshot.png, description, etc., and I CAN activate the theme, but it doesn't render a preview the way all my other themes do, just a blank page with no content or wp elements. The original theme I duplicated displays perfectly.
Does anyone know how to rectify this?