when exporting a page that contains a css with a background image, this latter doens't appear

Closing as won't fix since the old PDF export is now deprecated in favor or the more recent client-side PDF export, see https://forum.xwiki.org/t/deprecate-old-pdf-export-and-retire-it/13520. Thus, all issues except blocker issues are going to be closed as won't fix. Ofc, if anyone wants to fix this issue, they're free to go ahead and we'll reopen the issue.