Align theme toggle with system preference and react to prefers-color-scheme changes

[Copilot]

Theme selection now treats browser/OS preference as the default mode instead of always persisting manual toggles. When a user-selected theme matches prefers-color-scheme, the override is removed so future system changes are reflected automatically.

• Behavior model

  • Introduced explicit resolution flow:
    • stored theme (if different from system) wins
    • otherwise system preference wins
  • Added normalization to clear legacy/local overrides that equal current system preference.

• Toggle semantics

  • Updated toggle handler to persist only true overrides.
  • If the toggled value equals current system preference, localStorage.theme is removed (return to system-following mode).

• Live system theme updates

  • Added matchMedia('(prefers-color-scheme: dark)') change listener.
  • On system theme change, re-applies theme only when no explicit override is stored.

<script>
  var colorSchemeMediaQuery = window.matchMedia('(prefers-color-scheme: dark)');

  function getSystemTheme() {
    return colorSchemeMediaQuery.matches ? 'dark' : 'light';
  }

  function getTheme() {
    return localStorage.getItem('theme') || getSystemTheme();
  }

  document.getElementById('theme-toggle').addEventListener('click', function() {
    var next = document.documentElement.getAttribute('data-bs-theme') === 'dark' ? 'light' : 'dark';
    if (next === getSystemTheme()) localStorage.removeItem('theme');
    else localStorage.setItem('theme', next);
    applyTheme(getTheme());
  });

  colorSchemeMediaQuery.addEventListener('change', function() {
    if (!localStorage.getItem('theme')) applyTheme(getTheme());
  });
</script>

Reference: #174 (comment)

---

📊 Dashboard preview 📊: https://python-docs-translations.github.io/dashboard/186/merge/

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 0ed123f91a

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".