The Skins Factory has been a leader in desktop customization, delighting users from around the world for over 10 years so it was a "no-brainer" bringing theme customization to Zen Viewer. Zen Viewer ships with a default them and 5 alternate themes called The Light Terrestial Series. These are available as a free in-app download via the Theme Manager.
Also available via the Teme Manager is the Dark Terrestial Series - darker, variant versions of the Light Terrestial series themes that are perfect for night use. The Skins Factory & Hachisoft will be creating new and exciting themes for Zen Viewer in the months to come, so check back often.