I'd like to create new GXT 3 theme. I have several questions:
Does exist GXT 3 themes on market?
Which min screen size GXT supports? When painting components in graphical editor which area size I should select? 1024x768 ? 1280x1024 or anything else?
ContentPanel and Framed panel look like rounded rectangles. But they do not use css3 border-radius property. Instead of it use complicated html syntax and image slicing. Can I use css3 styles in my appearance of content panel and other widgets?
Is there examples of creating custom theme for GXT3 widgets like layouts, panels ?
I can't answer all of these, but I can try for the technical questions.
2) There is no 'minimum size', since even on a large screen the user can't be stopped from shrinking the browser window down to something very small. This is usually considered to be an application issue (i.e. rebuild the app for very very small sizes), or a user issue (i.e. stop making the window so darn small).
3) We currently use images as this ensures backward compatibility with all supported browsers - IE6, 7, 8 do not have rounded corner support. Likewise, all gradients are done with images - those same browsers don't support gradients, and IE9's gradient implementation is both very different and very slow.
The only thing preventing you from using CSS3 features in your own application is your need to support browsers which do not support those features. With the CssResource support for @if user.agent checks, you could provide two implementations, and know that only the code that is used in each browser would be compiled in.