Improve Toast appearance of "neutral" variant

[jossmac]

Implements better handling of "neutral" toast appearance, especially for dark mode.

Before	After

[changeset-bot]

🦋 Changeset detected

Latest commit: 57c9b52

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 14 packages

Name	Type
@keystar/ui	Patch
@keystar/docs	Patch
@example/next-app	Patch
@example/next-pages	Patch
keystatic-docs	Patch
@keystatic/core	Patch
@example/astro-content	Patch
@example/astro	Patch
@example/localization	Patch
@example/next-block-builder	Patch
@keystatic/remix-test-app	Patch
@keystatic/templates-astro	Patch
@keystatic/templates-nextjs	Patch
@keystatic/templates-remix	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[Copilot]

Pull Request Overview

This PR improves the visual appearance of the "neutral" variant of the Toast component, with specific enhancements for dark mode. The changes ensure better contrast and consistency in the toast's styling across different color schemes.

• Improves styling for neutral toast variant with better background and foreground color handling
• Adds dynamic color scheme detection to properly style action buttons and close buttons
• Replaces hardcoded divider color with a dynamic CSS custom property for better theme integration

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}