Maximum WordPress.org themes do not provide the option to remove the Copyright tag or Creator URL or Credit line etc. Sometimes they show a link to Theme Creator's website.
All WordPress.org themes are published under 'General Public License' (GNU GPL or GPL), so we have the right to edit the PHP code and delete the Credit line or Replace it with own text.
I can help you to remove or replace the 'Copyright tag' from 99.99% WordPress.org themes. Are you interested for personalized branding? Hire me!