Modals should have a box shadow

Problem/Motivation

Modals in the theme currently display without any kind of shadow or visual separation from the background.

Proposed resolution

Add a box-shadow to the modal. Actually, the existing code is meant to provide a box shadow based on properties provided in the --jui-dialog-box-shadow CSS variable, but currently this is undefined. So, this could be addressed by creating a value for this CSS variable.