Admin Menu: Sync styles from prototype

[obenland]

This is a follow-up to @cpapazoglou's comment here:

I suggest merging it and iterating more on:

• Mobile menu open / close
• Site Switcher menu item missing

—#17629 (review)

Changes proposed in this Pull Request:

• Sync CSS changes from wpcom proptype

Does this pull request change what data or activity we track or use?

No

Testing instructions:

• Go to '..'

Proposed changelog entry for your changes:

• None needed.

[matticbot]

Caution: This PR has changes that must be merged to WordPress.com
Hello obenland! These changes need to be synced to WordPress.com - If you 're an a11n, please commandeer and confirm D54046-code works as expected before merging this PR. Once this PR is merged, please commit the changes to WP.com. Thank you!
This revision will be updated with each commit to this PR

[jetpackbot]

Scheduled Jetpack release: December 10, 2020.
Scheduled code freeze: December 3, 2020

Thank you for the great PR description!

When this PR is ready for review, please apply the [Status] Needs Review label. If you are an a11n, please have someone from your team review the code if possible. The Jetpack team will also review this PR and merge it to be included in the next Jetpack release.

Generated by 🚫 dangerJS against d4e44b7

[obenland]

@cpapazoglou Could you please advise on which styles are needed and which can go (if any)? And if the Site Switcher issue you mentioned is still an issue?

[cpapazoglou]

We will probably iterate on the colors as a result of the color-schemes feature but right now LGTM! Let's merge it and start deploying in wpcom and deleting the duplicates found there so that we have a canonical reference! Thanks!

[obenland]

r218228-wpcom