Bug 29070: Accessibility OPAC Purchase suggestions box on search results page has...

Bug 29070: Accessibility OPAC Purchase suggestions box on search results page has insufficient contrast

This patch changes the text colour for the purchase
suggestion box on the OPAC search results page to
make the text easier to read and improve accessibility.

To test:

1. Log in to the OPAC and search for something
2. Use an accessibility tool (e.g. Chrome's
   Lighthouse tool) to check the contrast ratio
3. Confirm that the Purchase suggestion text
   fails to meet accessibility requirements
4. Apply the patch and rebuild CSS:
   yarn build --view opac
5. Confirm the text contrast ratio now meets
   minimum accessibility requirements
6. Check that the OPAC still 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>