What would I need to add?
 

What would I need to add to make the navbar start from the inside of the border to stretch to the other side?

Edit: Problem solved thanks for your advice :)

huh? please explain ...
to make things clear - upload a screenshot of what you want to fix

Sorry :)

$spacer_open is responsible for the width.
In older versions you could close with $spacer_close before the navbar and reopen with $spacer_open after the navbar.

This doesn't seem to work any longer in 3.7.
So I ask for any help too, because I would like to extend navbar and breadcrumb to the full width of header.

it's hard to tell in a custom style

if it's $spacer_open..
you can try removing it as well as $spacer_close from above and below navbar - you may have to edit the template to suit your needs