The branding of a DotNetNuke (DNN) website is carried out through the use of skins and containers.
A skin is a set of files that are used to define the "look and feel" of a page which also includes the menu navigation.
A skin can be applied to a whole portal website or an individual page.
A container is a set of files that are used to define the "look and feel" of a module on a page.
Each module on a page can have a different container.
There are many "free" and Open Source website templates available that can be used as the basis for DotNetNuke Skins and Containers.
Some useful references are shown below:
This product relates to the design of DotNetNuke Skins and Containers that are either entirely based on your own brand identity and to your brief and/or based on modifications to Open Source templates.