Bug 29126: Accessibility: More corrections to contrast in the OPAC

Bug 29126: Accessibility: More corrections to contrast in the OPAC

This patch makes a few more corrections to OPAC CSS in order to avoid
having elements which lack sufficient contrast, in particular parts of
the interface where a standard-colored link appears on a darker
background.

To test, apply the patch and rebuild the OPAC CSS
(https://wiki.koha-community.org/wiki/Working_with_SCSS_in_the_OPAC_and_staff_client)

Check the following pages/areas:

- Breadcrumbs navigation
- Datatables buttons (on the "Your summary" page)
- Bibliographic view tabs (Normal/MARC/ISBD) on the bibliographic detail
  pages.
- "Action" menu on the bibliographic detail pages: Place hold, print,
  etc.
- Links inside "striped" tables: Links on the alternate grey background
  are darker to increase contrast.
- Sidebar links on user pages: Your summary, your charges, etc.
- Tag cloud: Approved tags in the weighted list.

Signed-off-by: Jérémy Breuillard <jeremy.breuillard@biblibre.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>