Add background color to doc nav bar, to help with horizontal scrolling.

[emackey]

Fixes #9162.

My local doc build is busted, so I can't test this locally except by jamming it in DevTools.

Also I set the nav bar to be a slightly darker color, to draw more attention to the main panel.

[cesium-concierge]

Thanks for the pull request @emackey!

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[GatorScott]

Works well, but the horizontal scrollbar spans the entire window, not just the "content" side.

[emackey]

Yes, this is just a patch. Originally these things weren't supposed to horizontally scroll at all, just word-wrap more tightly. I'm not sure why some of them now have such wide minimum widths.

[OmarShehata]

Thanks @emackey . I think this color looks nice and it's a good quick fix since it doesn't look like a bug anymore.

Before:

After:

The reader can scroll horizontally to see the obscured text.