I feel as though the web-based versions are starting to look better than the Outlook 2013 product, which has gone from being fantastic in 2010 to being very flat and dull in 2013. I understand the point of Metro, of course, is to focus on content and less on the UI, but Outlook 2013 is lifeless and all the pale whites and grays just blend together and make it difficult to read anything. I actually uninstalled the entire Office 2013 preview, because I was so frustrated with it. I'm happily back on 2010.
Anyway, the point is that it seems like the themes and other color emphasis you've added to OWA makes it much cleaner to read, although the screenshots posted don't really do it justice. I'd love to see high-res versions. Just make sure that "clean" doesn't mean "lifeless" and hard to discern content from UI elements.