How can I have a node with the URL /themes I've created a node and have given it the URL /themes. In full: http://site-name.com/themes.

Permissions on the Drupal system folder 'themes' mean that the URL /themes gives: 403 forbidden.

Is there anyway to have a node with /themes? I'd prefer if possible to keep my current file structure as I think it would be difficult to change with my current host (Pantheon).
In your .htaccess file, the URL re-write has the a code to point the path to the respective folder, if present.. you can change that!