Header and footer are set under the theme. Headers and footers are just a widget defined under the theme settings.
You can define a page container as a subheader like this:
Service Portal uses Bootstrap layout by default. In order to add a widget, you must add first a container and then a row and a set of columns. The bootstrap will work for most of the cases but if you needto have a custom layout, for example you want to use flexbox instead of containers you can. Mark the bootstrap alternative option.
This option will get rid of the container class. Now you can add a CSS class to the container to have a different layout.