Well, got some time and refactored the themes tester app a bit, converted it to a themes editor 
The goal is to make a stand-alone, web based themes editor, in order to let non java-aware web designer to create/modify/preview new themes using only the web browser.
The app is now pushed to dev.vaadin.com SVN, into the contrib/vte
The live demo is here:
http://demo.stor-m.ru/VaadinThemesEditor
What’s new from the initial sketch:
- Overall look was polished a bit
- Main UI theme does not change, only the preview panel reflects the theme being previewed
- Theme export/import has been added
- New theme can be created from UI, standalone or from donor (base) theme
- Custom themes can be deleted
- App is now i18n aware (only English translations at the moment)
In plans to the next version:
- online css/resources manager to upload/download files and images, edit css, edit style definitions
- dashboards for previewing several widgets at once
- your ideas/thoughts ?