We use the slate theme in a large project. The customer likes it. See Link provided by @Nick Craver.
To date, I have found that it is not very easy to change the default appearance of more complex ExtJS components, such as grids or trees. However, one of the reasons I like ExtJS is to provide complex, capable components out of the box. Usually I stick to an existing topic when I plan to use these components.
Nothing interferes with the configuration of components, and ExtJS provides the low-level utilities needed to create something.
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