Bug 31973: Restore background color to message-style alerts

Bug 31973: Restore background color to message-style alerts

This patch restores the background color to message-style alerts, those
which use the markup <div class="dialog message">. These alerts should
stand out more to the user.

To test, apply the patch and rebuild the staff interface CSS.

- Test areas which use the message-style dialog. Perhaps the easiest
  example is to try to view a bibliographic record which doesn't exist:

  /cgi-bin/koha/catalogue/detail.pl?biblionumber=1234567890

- It can also be found as a confirmation when deleting a basket in
  Acquisitions, deleting a patron attribute type in Administration,
  updating a Z39.50 server in Administration, etc.

- Confirm that the alert looks correct and is suitable visible.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>