There's no way of editing a theme offline. But you can use the Textmate bundle to edit your theme inside TextMate (or E-editor on the PC) and the bundle then pushes you edits to your live shop using our API: Meeech's shopify.tmbundle.

Shopify now has multiple theme support, so you can make a copy (duplicate) of your theme on your Theme Manager page in your shop admin, and edit that unpublished theme. The Textmate bundle will soon support editing unpublished themes. Before it does, you can edit your unpublished themes online. See How do I edit my theme and preview my changes without affecting my live shop?