Añadir header | WordPress.org - Wordpress Tutorials, Tips, Themes and Plugins.

That’s not really a header, just a notification or banner

Oh thanks Steven, I just put it and worked perfectly 🙂

One other question, do you know how can I change the size of the menu text?
Marked in red here:
https://i.imgur.com/gKA07y7.png

Thanks again 🙂

main-navigation {
    font-size: 80%;
}

To add or override CSS: use the “Additional CSS” option in the customizer. https://wordpress.org/support/article/appearance-customize-screen/

Learn to use the Chrome Developer Tools or Firefox Developer Tools to help you see and test changes to your CSS.