tabs background

Not sure what you mean? Which tabs? Do you have a link to your site?

Yes sure, here is the page with the tab : http://iconink.fr/acp/bibliotheque/

Ah, you would typically want to ask the plugin author these kinds of questions, as those tabs aren't in GP.

However, this CSS might help:

.fl-tabs-label.fl-tab-active,
.fl-tabs-panels {
    background: transparent;
}

Would that be something from Beaver Builder plugin ?
I put the code you gave me in the advanced tabs setting, in the Class row, but it didn't work...

Yea it looks like it's from Beaver Builder.

You'll need to add it like this: https://generatepress.com/knowledgebase/adding-css/