Hi there,
any reason not to use the Header Element – which does that kind of thing?
https://docs.generatepress.com/article/header-element-overview/
If no good then use after_header
hook.
You Markup would look something like this:
<div class="green-wrap">
<div class="blue-container grid-container">
<!-- markup in here -->
</div>
</div>
The hook will automatically fit 100% wide.
The grid-container
class is a GP Class and will size the green container to match the sites container size.