try to reproduce the layout of the warble demo

hello, i've just installed the warble theme and created a child theme of it. I'm wondering about one thing. On the warble demo, you can see that there's the same background image covering the header and banner content. It's the same on internal pages, where the header and page title seem to have the same background, and it looks great. On my site https://chantillynegra.fr. I can't reproduce this. same for : https://chantillynegra.fr/boutique/ https://chantillynegra.fr/blog/ The header background is black and different from the content underneath. On your demo it's as if the header and block group below had the same background... I don't understand how you do it. thank you for your assistance Translated with DeepL.com (free version)