Updates navigation header background color

[ghost]

Description

Updates the background color of the header navigation. The same change is going to be made for the 1.3 branch as well as in the project website. This change uses a solid color instead of one with transparency, because the background colors of the body elements are different in the project website and the documentation site which makes a slightly darker tint in the header in the project website than the documentation site.

This also adds a kludge to get around outdated SCSS compilers not liking the min function with mixed units which has already been pushed to 1.3.

Corrects header logo link URL.

Checklist

• By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and subject to the Developers Certificate of Origin.
  For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[kolchfa-aws]

LGTM

[opensearch-trigger-bot]

The backport to 2.8 failed:

The process '/usr/bin/git' failed with exit code 128

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add ../.worktrees/backport-2.8 2.8
# Navigate to the new working tree
pushd ../.worktrees/backport-2.8
# Create a new branch
git switch --create backport/backport-5880-to-2.8
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 e845866e8eacd8d698b0084cfad183768e7b2b16
# Push it to GitHub
git push --set-upstream origin backport/backport-5880-to-2.8
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/backport-2.8

Then, create a pull request where the base branch is 2.8 and the compare/head branch is backport/backport-5880-to-2.8.