Make announcement bg darker and navbar bg semi-transparent

[chalin]

• Fixes Navbar items are very difficult to read in dark mode #54234
• Applies a filter to the #announcement so that the background color is a bit darker.
• Sets the navbar to be a little bit less transparent
• The above two tweaks make the navbar text readable
• Longer-term fix is to upgrade Docsy. It has improved navbar styling and reliable light/dark mode support

Preview: https://deploy-preview-54235--kubernetes-io-main-staging.netlify.app/

Screenshots

Before:

After:

Same solution applied to the standard hero:

/cc @nate-double-u

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