I recently helped Kateoplis put together a new theme for her site and not one day after putting in the final touches I encounter Tumblr Themr; an application designed to help you develop Tumblr themes offline. The instructions seem pretty clear as to how it works but it's a bit of a bummer that the Custom Fields don't work in the application - as that's one of the aspects of creating a Theme that make it more applicable to a wider audience. Regardless, this is likely just a first release and I look forward to seeing future versions make their way into the wild.