I created a feature request for this: https://stackideas.com/voices/easysocial/item/645-dark-mode-light-mode
However, this is something I would be highly interested in using. In fact, to keep things simple if the "Theme Switcher" you create for the demo site could be modified slightly to retain the users selection until changed. And, if the admin could choose which templates are included. I would pay for it now.
Please vote it up.