As some have noticed, the Seven theme (and I bet a lot of other themes out there) contains a pretty massive CSS reset, which sets most styles from system.css right back to zero.
One might ask, why not override system.css completely (by having a file with the same name inside the theme) instead of resetting everything?
I guess in most cases, the problem is that system.css contains a lot of 'functional' styles. Styles that are pretty much required in order to be able to use Drupal properly. Think of
body.drag, etc. These are all styles that themers probably don't want to touch.
However, not overriding system.css completely means you get some other, non-functional styles too, like the gray table backgrounds and lots and lots of margins and paddings. Themers don't seem to like these styles. They don't seem to want or need the default margins, paddings, borders and gray tables. (I'm also speaking for myself here btw :P)
Which results in a monster CSS reset, as happened with Seven.
The problem could be solved by seperating the 'functional' styles from the non-functional, 'stylish' styles. We have system.css and defaults.css already, so why not use system.css just for functional styles and defaults.css for the stylish default styles. This would make themers able to easily override defaults.css without having to override all the functional stuff too. Yay :)
I have attached a patch which simply moves some of the styles from system.css from defaults.css, no other changes. It's just a quickie, there might be some other styles that need to be moved around across these two files, but I'll await some feedback on this first.