We recommended you to remove unused themes. This is a good practice for protecting your WordPress website from vulnerabilities.
1- Login to your WordPress admin dashboard.
2- Under the dashboard menu, hover your mouse on Appearance and then click on Themes.
3- Hover your mouse on a theme which is not active and unused or you wish to remove that theme. It will show you the option Theme Details. Click it.
4- A simple window will open. At the bottom area, in the right side of the window, you can see the Delete option. Click it!
5- A confirmation window will open. Click OK to remove your theme.
The theme is successfully removed!