Regarding New Theme and layout

I need to implement a new layout for my site. I need to you suggestion, on what way I have to design the template, CSS and how to implement in Yii Framework.

My suggestion is to read this tutorial.

And some wiki topics here.

About implementation, there is nothing of special.

Layouts are in protected/views/layout.

In protected/views/layout/main.php you can include the css you need.