I only ran the demo, but it seems quite impressive with a lot of pre-built widgets (panels, layout, components ...).
Instead of making your own html/javascript code, you build your interface in Java and it generates all the code for you.

The framework is opensource (MPL/LGPL) and they provide you with an EchoStudio built on the Eclipse Framework, though you have to pay for this piece of software.

I haven't tested it (yet ?) but appearantly you can built custom components and, of course, use your own stylesheets.

Maybe it is something to try ...

Echo2 website

PS: Thanks to Lionel for the corrections