It's a good idea to delete inactive plugins and themes. However, when you upgrade WordPress core to a new version it automatically installs all default themes again.
So many people either leave those default themes on their website or waste their time deleting them after each core upgrade.
Luckily, it is possible to stop WordPress from installing default themes during an upgrade. You need to add the following code snippet to your wp-config.php file:
define('CORE_UPGRADE_SKIP_NEW_BUNDLED', true);Once you set this constant, WordPress will stop installing default themes.
If you're one of our managed WordPress hosting customers, you don't have to worry about this. We've already included this code snippet by default, so you shouldn't be seeing default themes at all. If you have any questions, contact support. We're happy to help.