La catégorie 'couleur de fond' ne semble pas évident qu'elle fait référence à la couleur du badge de la catégorie

[Rhidian (Rhidian Bramley)]

The category background and foreground colors are useful if you use the category header component.

See examples here:

Discourse Category Headers theme component

This theme component provides a number of enhancements for the Discourse category header. The ‘standard’ Discourse category header is displayed at the top of each category page, above the navigation links and list of topics. It is normally only visible if a category logo image has been uploaded in the category settings. The header displays the logo and a short description of the category, taken from the first paragraph of the ‘About the… category’ topic. This theme component provides the foll…