VuFind 2.0 only has one non-mobile theme, blueprint, which is on by default. The “root” theme is not a displayable theme – it just contains shared resources that are used by both the blueprint and jquerymobile themes. The old “classic” and “default” themes from 1.x have been deprecated and are not included in 2.0 (due to their reliance on YUI libraries, which made them a maintenance headache). We will soon be introducing a new alternate theme based on Bootstrap, which will likely be included with VuFind 2.2.
If you let us know exactly what you are hoping to achieve by changing the theme, perhaps we can help you in a different way….