Bug 29035: Accessibility: OPAC masthead_search label doesn't have sufficient contrast...

Bug 29035: Accessibility: OPAC masthead_search label doesn't have sufficient contrast ratio

This patch changes the colour of the masthead
search label to improve accessibility and make it
easier to read.

To test:
1. Use an accessibility tool (e.g. Chrome's
   Lighthouse tool) to test the contrast ratio.
2. Confirm that the search label has 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 now meets accessibility
   requirements.
5. 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>