Add WCAG AAA colour contrast option to theme editor

[delucis]

Description

• Adds a toggle to the Starlight theme editor to allow people to opt in to shades that pass WCAG’S AAA requirements (7:1 contrast ratio for body text).
• Preview: https://deploy-preview-2282--astro-starlight.netlify.app/guides/css-and-tailwind/#color-theme-editor
• The getPalettes() function used to generate a colour palette now takes a minimum contrast to enforce and uses it for the shades which are currently marginal (pass AA requirements but not AAA).
• Contrast calculations are done with culori’s wcagContrast() method. I used that to compute a contrast and then colours without sufficient contrast have their luminance incremented/decremented in 0.5% steps until the contrast requirement is met.

Browser testing

Tested on macOS with:

• Chrome 128.0.6613.113
• Firefox 129.0.2 and 130.0
• Safari 17.6

Tested on Android with:

• Firefox

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Latest commit: 1b2a4c4

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[astrobot-houston]

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Tracked Files

Locale	File	Note
en	guides/css-and-tailwind.mdx	Source changed, localizations will be marked as outdated.

Warnings reference

Icon	Description
🔄️	The source for this localization has been updated since the creation of this pull request, make sure all changes in the source have been applied.

[astrobot-houston]

size-limit report 📦

Path	Size
/index.html	6.15 KB (-0.02% 🔽)
/_astro/*.js	22.33 KB (+0.01% 🔺)
/_astro/*.css	13.72 KB (0%)

[HiDeoo]

What a lovely PR, really amazing addition to the theme editor 👏