> 1)  When I go to "edit design" command in Pier, all that Css I see, where it
> comes from ? I didn't see any Pier library that has those css.

The way you add styles is the Seaside way of doing it. That's fine and
works well with Pier. However, the styles in 'Edit Design' are part of
Pier itself, so these are editable from within Pier.

> 3) When I add different libraries to a Application, how Css are inherit ? If
> I have defined different thing for a same class in different libraries for
> example, what happens ?

It depends on the CSS selectors you use and on the order you define them:

1. CSS selectors have different weights
(http://www.w3.org/TR/CSS21/cascade.html). The more specific a
selector is, the higher its weight. With "!important" you can make a
definition override anything else, but I would only use this as a last
resort.

2. If the weight of several selectors is the same, it depends on the
order. Seaside libraries always come first, then the custom styles
from components and from Pier in the order they render.

> 5) I tried to put some css in mylibrary, like .a:hoover .a: link
> .component   but they are ignored. I guess it is using the css I see in
> "edit design". How can I do to override that css ?

Remove the style in "Edit Design" if you use your own CSS in a WAFileLibrary.

Or don't use the WAFileLibrary and replace the style in "Edit Desing"
with your own.