Hi Nathalie,
Thanks for reaching out! I’d be happy to help you with this.
It sounds like you added the CSS to the correct spot, but it may be getting overridden by other CSS rules later on. You could try adding an !important tag, or also adding your CSSĀ to the custom CSS area under Appearance > Customize (if your theme has one there) or into the style.css file of your child theme.
You can also send me the CSS you added and a link to the page where this is happening, and I’ll be happy to take a look!
Thanks,
Jennifer