In Pro Sites it makes sense to cover signup pages with SSL so passwords aren't passed in the clear. The issue is that if the signup pages are in SSL, the theme previews included in New Blog Templates are also in passed in SSL.
I recommend changing the New Blog Templates theme preview links to force http. If, for some reason, the sites being previewed are also in SSL they will automatically redirect back to SSL, so the isn't an issue. But this will fix the SSL error thrown when SSL is forced for non-SSL assets (i.e. theme previews when the signup pages are in SSL).
The simple fix is to change this in blogtemplates.php:
$blog_url = get_site_url( $template['blog_id'] );
$blog_url = get_site_url( $template['blog_id'], '', 'http' );