Add a stripe above menu

Hi there,

try adding this CSS:

#sticky-navigation,
#sticky-placeholder {
    border-top: 4px solid #f00;
}

That will apply to just the sticky nav.... if you want it also when the nav is not sticky, then can you share a link to the site so i can see the layout ?

What I need is something like it is on the library theme Cloud Built by Mike Oliver.

Is there any hook that I can use to include there some custom html?

You can use a Block Element - Hook to do that:

https://docs.generatepress.com/article/block-element-hook/

If you set the hook to: before_header it will appear in that position.