Please see
https://www.linuxquestions.org/questions/faq.php?faq=welcome
You haven't stated which version of XFCE, and "Debian 10" is not sufficiently precise - there are thirteen point releases.
Have you consulted the
XFCE Documentation yet?
What system control panels have you looked for settings in?
Have you checked the hidden config files in the user's directory?
Is this even definitely a problem - i.e. you haven't indicated if you spoke to the client about it and confirmed they want it fixed - perhaps they willingly made the change themselves...?