Hiya Matty!
Thanks for giving this suggestion! All of ideas given to us are super useful, as we get to see what your thoughts are and we can improve the server and gain new ideas!
Unfortunately, this suggestion has been declined. This is because it would take a lot of effort and would be difficult to implement with our current tech. The mail system that we have is a separate plugin that is coded. All of the mailboxes are linked to one menu automatically. Having a different background design would require additional coding and a separate GUI would have to be created. This would also need a lot of time to be designed and created. We have other priorities currently, so we will not be implementing this. We hope that this makes sense!