Bug 29038: Accessibility OPACUserSummary heading doesn't have sufficient contrast

Bug 29038: Accessibility OPACUserSummary heading doesn't have sufficient contrast

This patch changes the colour of the OPAC User Summary heading text to
increase the contrast ratio, improving accessibility and making the
text easier to read.

To test:

1. Log in to the OPAC homepage as a user with items checked out
2. Use an accessibility tool (e.g. Chrome's Lighthouse tool) to test the
   contrast ratio.
3. Confirm the UserSummary heading has insufficient contrast.
4. Apply the patch and rebuild css:
   yarn build --view opac
5. Use the accessibility tool to confirm the contrast ratio now meets
   accessibility requirements
6. Check that the OPAC still looks ok after the changes

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>