Replace horizontal paddings of modal headers with negative margins + increased paddings

[Vectrex]

This would allow for modals with colored background headers (e.g. confirm or alert dialogs) with a simple

<div class="modal-container">
  <div class="modal-header bg-primary text-light">
    ...
  </div>
  <div class="modal-body">
    ...
  </div>
</div>