If you have already tried to customize an Ext JS 3 theme, you know that it was a complicated task— and a bit annoying, too. You had to open Firebug, inspect every single element, go to the CSS file, and then change it to a new color/background, and so on. Some Components of Ext JS 3 were composed of some images, such as the Button Component. If we wanted to change the button color, we had to create new images and then apply them to the Ext JS 3 button.
All of these complications are no longer present in Ext JS 4. Version 4 introduces a new way to customize and create new themes, using CSS3 features.
Ext JS 4 uses Sass and Compass to create themes. If you take a look at the Ext JS 4 resources folder, you will see that there are folders and files that will help us to create and customize a theme (Ext JS 3 does not contain these files):
