Check If WordPress Theme Exists and WP_THEME_DIR?

One thing we want to do with WPBook is allow custom theme modification that won’t be overwritten by plugin upgrades. I’ve been looking into ways to check to see if a theme exists. If it does uses it if not use the default plugin theme from the plugin directory.

I know how to do a php check for “if file_exists” however, checking for all theme files seems a bit silly. What I’m wondering is if there is a way to check if the theme exists with at least a folder and index.php file.

Another thing I’ve been wondering, there is a WP_PLUGIN_DIR function but not WP_THEME_DIR function is there a workaround for this?

Any help on this issue would be great!