I'm a senior webmaster, with such experience webmastering (CSS, XHTML) Smarty, XML/XSL ASP.net and php.
I'm so surprised about the high amount of css files spread all along Piwik. It makes them really hard to manage when you try to build a theme, or even changing just a property on a specified class.
It's all I think, as you can see, related to scalability.
These were my opinions once I finished the spanish translation.
See you soon, men!
Thank you for your very interesting feedback :-)
Trac is not the place to discuss design and global feedback though.
Please post your message on the piwik-hackers mailing list (more info on http://dev.piwik.org)
I agree that there is a lot of possible improvements in the CSS and HTML and organization of files. You are welcome to fix the problems. We are indeed looking for developers who could help and be part of the team to work on these problems.
Please contact the piwik-hackers mailing list with your message.
Also, you can post patches to fix some of the problems you mention, and make sure you test all the changes using the [wiki:HowToTestUI] checklist (it is so easy to break the UI by changing a few CSS classes)
I hope we can work together to make piwik better :)