Fix colour contrast on confirmation banner
Fix colour contrast on confirmation banner

White on turquoise is not enough colour contrast to pass WCAG AA.

alphagov/notifications-admin,gov-cjwaszczuk/notifications-admin,alphagov/notifications-admin,gov-cjwaszczuk/notifications-admin,alphagov/notifications-admin,gov-cjwaszczuk/notifications-admin,gov-cjwaszczuk/notifications-admin,alphagov/notifications-admin