Webmethods Skin creation - Best practice

Hi,

we are currently creating custom skin for our portal, the visual editory that comes with wm Portal only let’s us change basic elements, but we need to customize more element that are defined in the base.css and also create new ones.

What is the best way to achieve this, do we need to create our own css file for new elements?

What is the purpose of the extended.css file ?

thanks

Stephane