No, there isn't an easy way to make ExtJS 4 use ExtJS 3 markup.
Note that in ExtJS 4 the markup depends on the browser. In IE6-8 it'll look a lot like it did in ExtJS 3. Only browsers that support advanced CSS features will use the simplified markup.
You should probably take a look at the Sass/Compass stuff if you want to do major theming changes like this. Trying to do it using CSS directly will probably prove a lot more difficult. Generally I've found styling windows using Sass reasonably simple but it obviously will depend on exactly what effect you're trying to achieve.