Updates colors for a11y contrast requirements

[atsansone]

Fixes #9427

Staged: https://tony-flutter-site--docs-6warcibu.web.app

This doesn't cover updates to the lexer colors. That will happen later.

These changes:

• Darkened text color to create the required contrast between body background. (Current 3.49:1, Needs 4.5:1)
• Darkened background color of button to contrast with link text. (Current 3.79:1, Needs 4.5:1)
• Darkened text color of code text to contrast with the background. (Current 3.99:1, Needs 4.5:1)
• Darkened text color of copy to contrast with the link text. (Current 2.54, Needs 3:1)
• Change heading font settings to Design Standards.

Current:

Changed:

[atsansone]

@mit-mit : This is consistent with the Flutter branding guidelines. I would appreciate your approval.

[parlough]

I haven't taken a deep look yet, but the change results in a much darker background color on the banner which doesn't fit the style of the site or Flutter's lighter color scheme. It's also hard to distinguish the new link text color on the banner from non-links. Consider exploring other options for at least the banner.

[atsansone]

It's consistent with the branding. We should wait for @mit-mit to comment.

[parlough]

I'm happy to wait for others' thoughts. Can you update the staged link? It seems to not have these changes anymore.

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For more on my earlier comments: It's more than just applying a brand color though. The brand guidelines suggest Flutter leans towards the lighter side and Dart leans towards the darker side, but with this change, the first major color users see on this site would be one of the darkest colors, especially since this color makes it particularly stand out from the rest of the page.

And whatever color is chosen, the larger concern that should be addressed is the reduced contrast/difference in the banner between the text and the non-link text colors. I'd be happy to try out or discuss other options with you.

[parlough]

Thanks for adjusting the banner! It's still a bit too dark for the overall Flutter branding, but we can go with it for now to get the other fixes closer to landing.

One other issue with the sidenav border colors remains though:

[mit-mit]

Thanks for adding the description; this LGTM