fix(site): restore correct --sl-color-black in light mode for search box contrast

[Copilot]

Summary

Starlight maps --pagefind-ui-background: var(--sl-color-black) for the search modal. In Starlight's theming system the gray scale is inverted between modes, so --sl-color-black must be #ffffff in light mode (the background endpoint). The custom CSS was setting it to #1f2328 (near-black) — same value as --sl-color-text — producing an illegible black-on-black search modal in light mode.

Fix: one-line change in site/src/styles/custom.css:

/* before */
[data-theme='light'] {
  --sl-color-black: #1f2328;  /* ← used as pagefind modal background */
  --sl-color-text:  #1f2328;
}

/* after */
[data-theme='light'] {
  --sl-color-black: #ffffff;  /* matches Starlight's expected inverted value */
  --sl-color-text:  #1f2328;
}

--sl-color-text is unchanged; body text contrast is unaffected.

Test plan

Verified locally by inspecting the compiled CSS variable resolution: --pagefind-ui-background now resolves to #ffffff in light mode, with --pagefind-ui-text resolving via --sl-color-gray-2 to hsl(224, 10%, 23%) — correct contrast.