[Repo Assist] Migrate theme colors to CSS light-dark() function

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Closes #1004

As requested by @nojaf in #1004.

Summary

Migrates all theme color variables from a separate [data-theme=dark] override block to CSS light-dark(light-value, dark-value) syntax in :root. This is a clean, self-contained refactor that:

• Reduces the CSS size by ~50 lines
• Makes color intent explicit at the point of declaration (both values in one place)
• Automatically honours prefers-color-scheme without any JS when no user preference is saved

Changes

docs/content/fsdocs-default.css

• Add color-scheme: light dark; to :root so the browser knows the page supports both schemes
• Replace 25 color variables that differed between light and dark with light-dark(light-val, dark-val) inline
• Remove the 43-variable [data-theme=dark] override block; replace with a minimal:

  [data-theme=dark] { color-scheme: dark; }
  [data-theme=light] { color-scheme: light; }

  The color-scheme property on the root element drives light-dark() resolution throughout the page.
• Keep the existing [data-theme=dark] { ::backdrop { opacity: 0.75 } } rule unchanged (non-color properties can't use light-dark())

docs/content/fsdocs-theme-set-dark.js

Simplify the early-load script:

• Before: Read localStorage + matchMedia("@media (prefers-color-scheme: dark)") (the @media prefix was actually a bug; matchMedia doesn't use it), then only set data-theme=dark for dark preference
• After: If localStorage has a saved preference, set data-theme to that value. If no preference, do nothing — CSS light-dark() with color-scheme: light dark will automatically follow the OS preference with zero JS overhead.

RELEASE_NOTES.md

Added entry under ## [Unreleased] / ### Changed.

Behaviour

Scenario	Before	After
User has no saved preference, OS is light	:root — light colours	:root with color-scheme: light dark — light-dark() resolves to light ✅
User has no saved preference, OS is dark	JS reads media query → sets data-theme=dark → dark colours	CSS light-dark() resolves to dark automatically ✅ (no JS needed)
User explicitly chose dark	data-theme=dark set → CSS overrides	data-theme=dark set → color-scheme: dark → light-dark() resolves to dark ✅
User explicitly chose light, OS is dark	data-theme not set (only dark was set) → light colours	data-theme=light set → color-scheme: light → light-dark() resolves to light ✅

The last row is actually an improvement: previously if a user explicitly saved 'light' theme but their OS switched to dark, the old code wouldn't set data-theme at all, which with light-dark() would have defaulted to OS dark. The new code sets data-theme=light explicitly, overriding the OS preference as intended.

Test Status

This is a CSS and minor JS change with no F# code impact.

• ✅ No build artefacts changed
• ✅ Visual verification needed: test light/dark toggle, system preference, and localStorage persistence

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