Please add .modal-footer background color variable

Created by: thewebartisan7

Prerequisites

• I have searched for duplicate or closed feature requests
• I have read the contributing guidelines

Proposal

Please to add a variable for set background color for modal-footer. I know we can do so with utilities classes, but it would be great to also set this via variable, which by default could be just null so unset.

Thanks

Motivation and context

A modal-footer with light gray background without top border just looks nicer.