Bug 20622: Add some color to bootstrap modal headers and footers

Bug 20622: Add some color to bootstrap modal headers and footers

Right now the bootstrap modal header, body and footer are all white. It would be nice to give them a bit of color.

Test Plan:
1) Apply this patch
2) Find a patron, click the 'add message' button
3) Note the modal header and footer now have a background color

Signed-off-by: Kyle M Hall <kyle@bywatetsolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>