When using the new theme selector in 2.1, the sequence works like this:

1. Click on 'select default theme' button.
2. Select a theme.
3. Annoying message appears with theme description; click Continue.
4. Swear vociferously at computer as it has taken you back to the admin root page instead of to the theme selector, where you now need to be because you need to select at least two themes (default and tablet, which should basically always be the same).

I would suggest that on selecting a theme, you should be returned to the page that shows selected themes for each device type. (Ideally straight away, but if there is some purpose to the theme description page, then OK, from the continue button on that.)