Is it possible to use CSS code to change label on feature ribbon?
Not that I’m aware of. At least not an easy way if one isn’t really familiar with CSS and PHP.
If I can not change /DT9/_directory/template/content-listing.php directly
You can edit them if you really wanted, it’s just not recommended since it’s a part of the core theme directory and will be overwritten on each theme update. You will just need to re-customize the files after each update.