Update dark experimental theme for top bar buttons

[sophschneider]

WHY are these changes introduced?

Part of https://github.com/Shopify/polaris-backlog/issues/1573

WHAT is this pull request doing?

Updates from this figma for the dark-experimental theme

How to 🎩

Check out new tokens consumed in the top bar. Open the user menu to see new divider color and white alphas

https://admin.web.web-minimal-m4pm.sophie-schneider.us.spin.dev/store/shop1

[sophschneider]

/snapit

[github-actions[bot]]

🫰✨ Thanks @sophschneider! Your snapshots have been published to npm.

Test the snapshots by updating your package.json with the newly published versions:

"@shopify/polaris-migrator": "0.0.0-snapshot-20240506013707",
"@shopify/polaris": "0.0.0-snapshot-20240506013707",
"@shopify/polaris-tokens": "0.0.0-snapshot-20240506013707",
"@shopify/stylelint-polaris": "0.0.0-snapshot-20240506013707"

[sophschneider]

/snapit

[github-actions[bot]]

🫰✨ Thanks @sophschneider! Your snapshots have been published to npm.

Test the snapshots by updating your package.json with the newly published versions:

"@shopify/polaris-migrator": "0.0.0-snapshot-20240506190227",
"@shopify/polaris": "0.0.0-snapshot-20240506190227",
"@shopify/polaris-tokens": "0.0.0-snapshot-20240506190227",
"@shopify/stylelint-polaris": "0.0.0-snapshot-20240506190227"