Update styles for userbar and a11y checker in dark mode

[albinazs]

As part of #10418

Update styles for userbar and a11y checker in dark mode:

• Dark theme applied for the userbar (except for the trigger button) and a11y checker (pass missing context information for the dialog element via fragment)
• Few tweaks for a11y checker's as-is dark mode implementation (close icon, selector button)

Light theme is not affected

Before	After

Please check the following:

• Do the tests still pass?¹
• Does the code comply with the style guide?
  • Run make lint from the Wagtail root.
• For Python changes: Have you added tests to cover the new/fixed behaviour?
• For front-end changes: Did you test on all of Wagtail’s supported environments?²
  • Please list the exact browser and operating system versions you tested:
    Chrome 113, Firefox 113, Safari 16.3 macOS 13.2
  • Please list which assistive technologies ³ you tested:
    Color contrast check
• For new features: Has the documentation been updated accordingly?

Please describe additional details for testing this change.

Footnotes

1. Development Testing ↩

2. Browser and device support ↩

3. Accessibility Target ↩

[squash-labs]

Manage this branch in Squash

Test this branch here: https://albinazsfeature10418-userbar-d-efodt.squash.io

[lb-]

Looks great, thank you!