Unused themes should be removed. It is recommended that you do not preserve old, unused themes on your WordPress website to protect it from vulnerabilities.
1. Log in to the WordPress dashboard.
2. From the Dashboard menu, go to Appearance and click on Themes.
3. Hover your mouse on a theme you want to remove. Click on Theme Details.
4. Click on Delete and Confirm with OK. You can find the Delete option at the bottom right area of the window.
You have successfully removed the theme.