DNNs layout is based on a skinning system.

The layout of all pages on your site is controlled by the skin you choose for your site or possibly for different pages on your site.

The skin is made up of a set of layout files and skin.css files.  The layout files define the core design structure for your site and the css files define the styles utilized by that skin.

In addition - each module in your site is displayed with a container wrapper around it - each module can have a unique container wrapper. 
Each container can have its own layout files and container.css files.

Finally, individual modules can override the css files on the skin and container for the elements displayed by the module - these can be define in a module.css file.

Westa