Hi
I’m a senior webmaster, with such experience webmastering (CSS, XHTML) Smarty, XML/XSL ASP.net and php.

Some stuff:
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.

Some ideas:
- There are too much ID selectors… it increases substantially the memory used: and, in most of these cases, they could be identified as classes. There is too much repeated code on css. For example, at datatable.css, on line 173: #sortIconContainer{ float: right;}. It was really needful to create an ID just to add a floating?? And why an ID?? Why don’t we create a class, and add it anywhere we need it, not just here?
- The use of techniques as ‘top:-999em;’ are useless. This is more a hack than a technique. Solving cross-browser bugs can ask for hacks on IE6 or IE7, but as a last option. While something can be done with normal values, it shouldn’t be solved with a hack. I don’t say that: W3C says.
- The CSS uses quite too much the z-index property. It has never been useful to me, nor to my gurus: everything can be done without z-index. The .nav menu is a nice example.
- I really trust there should be an unique css file. By now, when you request a new query to the server, it loads too a css file or embedded css code, to handle the downloaded data. It makes the needful answer (the data) to late a bit more. I know…. just a bit… but bit by bit… we have a Byte…
- Piwik (should) need a webmaster to handle the common efforts. Is there anyone over there? Could I join the team please?
- I think the best way to do this is a MVC (Model-View-Controller) work-scheme: the style issues separated of programming issues. I have seen a lot of .tpl files with an embedded style label inside… Senior programmers can understand what I mean: {literal} for Smarty should not be used for this. There are better ways.
- The themes are really difficult to develope: look at wordpress: the active theme is stored as a folder, inside a ‘themes’ folder. The active theme is the one that includes all the templates, css, images, etc… inside its folder. Everything you can play with (as webmaster) is hold on that folder.
- I think that, if we are developing a webmaster focused tool, we must reach their way to see a webpage. Piwik loses credibility: everything must look attractive (charts are so beautiful, though), because it’s focus are persons who make things attractive. And even, if piwik can be watched/managed by more than 1 user, it must be easy for companies and their webmasters to convert everything to their colors, and include their logo, just playing with .tpl and .css files, as well as the images provided defaultly.
- If the flash charts are developed by someone, and are not done by a third party company, the .swf files could include the .fla files. Ain’t this Open Source?

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!