Productionize the High-Contrast mode POC that @cee-chen put together here.

The basic requirement for this is to support the this effort from the Shared UX team. We will clean up and productionize our POC and hand it over to the Shared UX team to:

• Integrate into Kibana
• Validate it meets A11y contrast ratio requirements

From the EUI side:

• We should mark this as a "Beta" feature, so we have flexibility to change it should it not meet Shared UX requirements
• We should ensure it is "opt-in", it should not be the default in EUI. We should provide enough flexibility so that Shared UX can implement it in a few different ways:
  • In specific locations in the the Kibana UI with a theme override relying entirely on the prefers-contrast media query
  • Wholistically at the top-level theme provider, enabled via a user toggle and/or the prefers-contrast media query
• This needs to work in both Amsterdam and Borealis