Use semantic color name instead of absolute color name

[dhruvkb]

Description

• Closes Right sidebar border does not respect hairline color #2790
• This PR updates the right sidebar to use the semantic color name (--sl-color-hairline) instead of an absolute color name (--sl-color-gray-6).
• This means that when a theme customises the hairline colors, the right sidebar will be automatically use that.

[changeset-bot]

🦋 Changeset detected

Latest commit: 88ded8a

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

This PR includes changesets to release 1 package

Name	Type
@astrojs/starlight	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

[netlify]

✅ Deploy Preview for astro-starlight ready!

Name	Link
🔨 Latest commit	88ded8a
🔍 Latest deploy log	https://app.netlify.com/sites/astro-starlight/deploys/6784e48cb1a18900081bfd71
😎 Deploy Preview	https://deploy-preview-2792--astro-starlight.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.
Lighthouse	1 paths audited Performance: 100 (no change from production) Accessibility: 100 (no change from production) Best Practices: 100 (no change from production) SEO: 100 (no change from production) PWA: - View the detailed breakdown and full score reports

---

To edit notification comments on pull requests, go to your Netlify site configuration.

[trueberryless]

LGTM 🎉

I think that we need to write a changeset for that. @dhruvkb Do you know how to do that? You can click the link in the changeset comment and describe the changes like that for example:

---
"@astrojs/starlight": patch
---

Uses hairline color variable for sidebar border.

[dhruvkb]

Cool, I'll look into the docs and write one asap.

[trueberryless]

Nice, I think it is documented here: https://contribute.docs.astro.build/docs-for-code-changes/changesets/

[delucis]

Thanks @dhruvkb! I’ll add a changeset so we can merge this.

(For future reference, you can do this by running pnpm changeset in the repo, or using the handy “Add changeset” link in the comment left on the PR by the changeset bot. But no worries, I’m always happy to add one myself if needed.)