Bug 29036: Accessibility OPAC buttons don't have sufficient contrast

Bug 29036: Accessibility OPAC buttons don't have sufficient contrast

This patch changed the base colour of OPAC buttons to increase the
contrast ratio, improving accessibility and making the button text
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 OPAC buttons don't have sufficient colour contrast
3. Apply the patch and rebuild css:
   yarn build --view opac
4. Use the accessibility tool to confirm the button contrast ratio now
   meets minimum accessibility requirements
5. Check that the OPAC still looks ok after the changes

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: jeremy breuillard <jeremy.breuillard@biblibre.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>