GitLab UI integration branch for jeldergl-pop-dark-tokens

What does this MR do and why?

Create an integration branch to test dark design tokens for tooltip, toast, and popover.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

See gitlab-ui!4479 (closed) for expected results.

How to set up and validate locally

• Hover over items with a tooltip, like an icon button.
• Click an element that opens a popover, like a user avatar.
• Perform an action that creates a toast message, like updating your status.