If you have installed a Moodle theme or plugin which is not compatible with your Moodle version or no longer needed you can completely remove it from your server and database by following the steps below.
Log into your Moodle site as an admin user, and navigate to the Plugins overview page by going to
Site administration > Plugins > Plugins overview
Find the theme or plugin you’d like to uninstall and click the link “uninstall”.
Uninstalling a theme or plugin will delete EVERYTHING (eg. theme configurations and plugin settings) in the database associated with the theme/plugin.
Manually remove the Moodle theme or plugin folder from your server. This step is important because if you don’t remove the folder from your server Moodle will detect the folder again and ask you to install the theme/plugin next time you visit your Moodle site’s admin page.