Delete themes: Old themes that are no longer needed can (should) be completely deleted from the WordPress installation.
This can easily be done via the admin administration in WP under the tab Design and then Themes. The following view shows the saved themes of the Wodpress installation. The Delete button is a little hidden. To do this, click on the Details button in the respective inactive theme and then activate the Delete button in the subsequently opened theme overview at the bottom right. Done, that’s it. The entire content of the no longer required theme on the server in the respective WP installation is also deleted. A separate deletion via FTP is no longer necessary.
This way you keep your WP installation slim and separate from unnecessary data waste.