Docs: fix Shiki dark mode rendering

[julien-deramond]

Closes #42566

Description

In dark mode, structural tokens inside certain code blocks were rendered as near-black and therefore invisible against the dark background. This was specifically visible in the <script type="importmap"> example on the Install page, where the JSON structure characters ({, }, :) would disappear.

The site has two code rendering paths:

• <Code> tab components — use highlight.ts, which already passes defaultColor: 'light-dark()' to Shiki.
• Markdown fenced code blocks — use Astro's built-in Shiki integration configured in astro.config.ts, which was missing defaultColor: 'light-dark()'.

Without it, Shiki defaults to defaultColor: 'light', generating color: var(--shiki-light) for all uncolored/structural tokens. The Bootstrap VS Code light theme's editor.foreground is #0b0c0d (near-black), which is fine on a light background but invisible in dark mode.

Explicitly colored tokens (strings, property names, tags) were unaffected because their spans carry both --shiki-light and --shiki-dark CSS variables. Only tokens falling back to the default foreground were broken.

The fix is rather simple: add defaultColor: 'light-dark()' to the shikiConfig in astro.config.ts, consistent with highlight.ts.

With this option, Shiki emits color: light-dark(var(--shiki-light), var(--shiki-dark)) for default-colored tokens. The CSS light-dark() function reads the color-scheme property, which Bootstrap already sets to dark on [data-bs-theme="dark"], so the correct dark foreground (#f6f7f8) is applied automatically.

New rendering:

Before the fix	Now

Live previews

• https://deploy-preview-42591--twbs-bootstrap.netlify.app/docs/6.0/getting-started/install/#cdn