CMS style theme and page layouts

I am new to yii but not php. I am planning a project that needs switchable themes with multiple page layouts in each theme. Concrete5 would be one example CMS with this setup.

My question is, how much of this is already basically done? How do you think I should use layouts, views, etc to implement this?

Thanks in advance

Well, you can easily switch themes in Yii in runtime, using CWebApplication::setTheme.

You can use same views, and different css files, in case your theme are slightly different(for example colors only). In other cases you will need to create different views for every theme.