Bug 20432: Add black version of small Koha logo for use in the OPAC

Bug 20432: Add black version of small Koha logo for use in the OPAC

This patch adds a black version of the Koha logo for use in OPAC
customization.

To test, apply the patch and add this CSS to your OPACUserCSS system
preference:

.navbar-inverse .navbar-inner {
    background: #CCC none;
}

 #logo {
    background: transparent url(/opac-tmpl/bootstrap/images/koha-logo-navbar-black.png) no-repeat scroll 0;
}

Refresh your OPAC and the top navigation bar should be changed from
black to grey, and you should see a black version of the Koha logo.

Signed-off-by: Séverine QUEUNE <severine.queune@bulac.fr>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>