WordPress is awesome in its allowing for choosing a theme you wish to use. However, having themes on your site you are not using can open you up to security issues. So let's see what we can do to minimize the security issues.
- Remove any themes you are not using. Keep in mind, your current theme might be a child theme, so be sure to not remove the parent theme.
- Update any themes you plan to use. Keep in mind, if there were customizations made to a theme, you will lose them on update. Best practice is to use a child theme.
Themes can be seen through the Appearance menu, themes options inside the dashboard.