Bug 29034: Accessibility: OPAC nav links don't have sufficient contrast ratio

Bug 29034: Accessibility: OPAC nav links don't have sufficient contrast ratio

This patch changes the colour of the navigation
links on the OPAC (Cart, Lists etc.) to improve
the contrast ration and make them easier to read.

To test:
1. Use an accessibility tool (e.g. Chrome's
   Lighthouse tool) to check the contrast
   ratio on the OPAC
2. Confirm that the navigation links have
   insufficient contrast
3. Apply the patch and rebuild the OPAC css:
   yarn build --view opac
4. Use the accessibility tool to confirm
   the contrast ratio meets  the minimum
   accessibility requirements
5. Check that the OPAC looks ok after the
   changes

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>