@bernatns – looks like your site https://ralvet.net/en/ is hosted over at WordPress.com.
We are aware of an issue with WordPress.com’s Custom Colours feature when using child themes, but we don’t have a fix yet. In the meantime, you can add this, custom CSS manually in your Customizer’s Additional CSS panel, or in your child theme’s stylesheet:
.site-header:before {
background-color: transparent;
}
I also noticed this note in your custom CSS area:
And to hide the thin gray line:
Just a heads-up that you need to add comment tags around this line, otherwise it may be preventing some of your CSS from working:
/* And to hide the thin gray line: */
I would suggest adding custom CSS either to your Customizer’s CSS panel or your child theme’s stylesheet, but not both. This avoids confusion and keeps everything in one place, making it easier to make changes and understand your code in the future.
Finally, just a heads-up that these forums are for folks running the WordPress software on their own hosting. Since you have a WordPress.com Business plan, you are entitled to live-chat and email support, which you can access here when logged into your account:
https://wordpress.com/help/contact
