I wouldn't delete it but the theme itself is "disabled" per se by enabling another theme in the theme manager, setting another theme as default. There are also widgets that go to Serenity. If you want to disable all of that, you can do that in the plugin manager. I have disabled it myself in order to create a new theme for SE. It seems to work fine for me. However, always test things on a development clone of your site. We do allow one development site. It must be in a development subdomain (named development) and closed to the public either by password protecting the directory or by having the site in maintenance mode which requires a code to access.