Exploring the Basics of Creating a Theme
Keep in mind that the more you know, the better you’ll be able to design and develop your own Tumblr themes — or at least get familiar with the structure of existing themes in Tumblr’s Theme Garden.
Gathering the tools you need
Before you develop or build your theme, you need to come up with a design for it. That involves design skills and working knowledge of one or more drawing, web-graphic, and photo-editing software programs. ...