Hi there,
1. You can use a Header Element, it doesn’t require any hero content, just select the Merge with Content on the Site Header tab
https://docs.generatepress.com/article/header-element-overview/
With that option enabled you are provided the choices to change site header / navigation colors and replace the Logo.
2. The default width is set in Customizer > Layout > Container. This should be set to maximum contained width you require ( Full Width options override this ).
Then if you want certain pages/posts to have a smaller width you can use the Layout Element:
https://docs.generatepress.com/article/layout-element-overview/