Boris,
so just changing .css my interface will dramaticly change ?
Yes, if you change it dramatically 😊
I may want to make it colorfull or insert some images... where can I touch the master structure to make a master like this openacs.org site ?
You can change the master template adp file if you want to, but mostly you should change appearance by just editing the css files. You can add images that are part of the layout to your pages by using css background property. That way you separate your page structure from the presentation and it's a whole lot easier to change the appearance later on.
and what about the other .css pages that aren´t dotlrn-master.css, what they´re about ?, from different packages ?
Yes, different package can define their own css files to overwrite "global" css rules or to add new ones.
And which webdeveloper should I use ?
Malte probably meant Firefox Web Developer Extension. It's a great resource when tweaking your layouts.
