Design a consistent color theme for the website

## Is your feature request related to a problem? Please describe.

The website needs a more intentional and consistent color theme across the landing page and regular documentation pages.

PR #5317 changed the shared navbar to a dark teal that works well with the landing page hero. However, that darker color can contrast too strongly with the white background used on regular pages. Using the previous lighter blue on documentation pages would soften that transition, but having different navbar colors on different pages may make the site feel inconsistent.

## Additional context

This discussion came from the review of PR #5317:

- [Initial navbar color feedback](https://github.com/apache/polaris/pull/5317#issuecomment-5343131954)
- [Follow-up discussion about the darker global navbar](https://github.com/apache/polaris/pull/5317#issuecomment-5371187095)

PR #5317 was merged first so that the homepage improvements were not blocked on a broader website theme redesign.

Start by reviewing PR #5317 and its linked navbar color discussions. Compare the dark teal used on the landing page with the lighter blue previously used on documentation pages, then define a consistent theme for both page types. Done means the website has an agreed, intentional color treatment without the current contrast or inconsistency concerns.

Written by the indexing model from the issue text.