Hi Yvon,
I would copy an existing styleshout skin and use it as a starting point.
The main thing to realize is that css is basically comprised of
selector {style rules}
in mojoportal you will mostly have to use different selectors than what the template uses based on css classes available in markup rendered by mojoportal when you view the page, so the idea is to harvest the style rules and put them in mojoportal appropriate selectors.
you do have some control of the outer wrapping divs and css classes in layout.master but most other things are determined by the cms but it does provide plenty of class names that can used in your selectors, you may need to view the source of the page and study the markup and classes to figure out the selector you need.
I will say that most of the styleshout skins have the same selectors but different style rules, so starting with an existing styleshout skin should have a lot fo the correct selectors already there and you just have to replace the style rules based on the new design.
Hope it helps,
Joe