Keep in mind: You should always remove unused themes. Not keeping old, unused themes is considered good practice if you want to protect your WordPress website from vulnerabilities.
Here's how you can delete an old theme.
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.
That's how you delete a theme.
