remove empty space on header

Add this code on the Custom CSS option:

#header {min-height: 0;}

https://ronangelo.com/frontier-documentation/

Thanks that worked.
If I don’t want to use the custom CSS option, what must I add onmy child theme, that will have the same effect?

I’m asking because I tried using what you provided, but it didnt work.

what must I add onmy child theme, that will have the same effect?

You add the same thing on the child-theme’s style.css

I’m asking because I tried using what you provided, but it didnt work.

I’m looking at your site right now and the css code is being applied correctly.