This Screencast is most relevant to Sencha Touch, 2.x.
Drew Neil (@nelstrom) shows how to theme Sencha Touch. This tutorial is perfect for beginners who want to get started customizing the interface of their Sencha Touch mobile app using CSS3 and Sass/SCSS.
Here's the source code on GitHub: github.com/senchalearn/Touch-Theming.
View live demo: senchalearn.github.com/Touch-Theming/.