How are css classes set for page sections, in this case the header section?

How do I set or update a css class for the entire header section of the page that contains our nav, site banner etc?

Is there something i can change that will add predefined classes?

The issue i am having is, in our dev env, the header section has a class of content which is causing unwanted padding but in our stg env there is a class of header-top-content  which has the appropriate padding.

I have tried editing the page, selecting header and updating the layout and looking in this section for somewhere to update the class but i am unable to locate it. 

Thanks,

Justin