Sharing: Improve contrast for accessibility.

[beaulebens]

Bumps accessibility rating for sharing buttons to WCAG AA & AAA Text
Props https://titan.as for submission via email.

Fixes #3227

Changes proposed in this Pull Request:

• Darken the text/icons used in sharing buttons to #545454 to improve accessibility.

Testing instructions:

• Enable Sharing buttons and add some to a post via Settings> Sharing
• Make sure you're using "Icon + text" or "Text only" as the button style option
• View a post and observe sharing buttons.

Before:

After:

Proposed changelog entry for your changes:

• Improve accessibility of sharing buttons by increasing contrast ratio. Props https://titan.as

[matticbot]

Caution: This PR has changes that must be merged to WordPress.com
Hello beaulebens! These changes need to be synced to WordPress.com - If you 're an a11n, please commandeer, review, and approve D27241-code before merging this PR. Thank you!

[jetpackbot]

	Warnings
⚠️	The PR is missing at least one [Status] label. Suggestions: [Status] In Progress, [Status] Needs Review

This is an automated check which relies on PULL_REQUEST_TEMPLATE. We encourage you to follow that template as it helps Jetpack maintainers do their job. If you think 'Testing instructions' or 'Proposed changelog entry' are not needed for your PR - please explain why you think so. Thanks for cooperation 🤖

Generated by 🚫 dangerJS against 5be0577

[eliorivero]

🐑
Code is good, needs design review and it can be merged.

[MichaelArestad]

Looks great!