Updates header navigation background color

[ghost]

Description

Updates the background color of the header navigation. The same change is going to be made for the main 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.

Corrects header logo link URL

Checklist

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