@Jeffro -

Hey Chip, long time no see :)

Likewise! Good to see you writing again.

It seems at this point, theme authors should be encouraged to have their own demo site that is accessible when you visit the theme authors home page as the .org previewer can not be guaranteed to show the theme the way it was meant to be viewed.

There is actually a third, and probably better, alternative: the Theme Customizer. Just install a Theme, then go to Appearance -> Themes, and click "Live Preview" for any installed Theme. That will initiate the Theme Customizer, allowing you to preview that Theme using your own site's content, and to manipulate that Theme's options (assuming it has hooked its options into the Customizer API), all without ever activating the Theme. If you like what you see: activate it. If not, just close the Customizer without saving changes, and then delete the Theme (if you'd like).