No automated way, nor can there even be a full and reliable enumeration as it is possible that e.g. a separate front end server serves some of the themes while forwards others to the Vaadin servlet which serves some from various JARs, some directly from the WAR etc.
The themes that come with Vaadin 6.7.x are “reindeer” (default), “chameleon”, “runo” and “liferay” (for Liferay portals).
In addition, there is the theme “base” which is not usable as is but rather used as a basis when developing a theme from scratch, not based on the above mentioned themes. This requires considerable effort, though.