Follow system theme on change when respect-user-color-scheme: true

[rmvegasm]

Problem

When respect-user-color-scheme: true is set in a Quarto HTML document, the
page respects the user's OS-level light/dark preference at load time, but does
not follow it when the OS theme changes while the page is open. This
behaviour is described in #12426 but I believe is no longer needed, and the
expected behaviour for most users setting respect-user-color-scheme: true
would be for the document to follow the system theme anytime it changes.

Root Cause

The matchMedia('(prefers-color-scheme: dark)') change listener in
quarto-html-before-body.ejs bails out early when any localStorage entry
exists:

queryPrefersDark.addEventListener("change", e => {
  if(window.localStorage.getItem("quarto-color-scheme") !== null)
    return;                       // ← bails forever after one manual toggle
  // ... apply system theme ...
});

Since every manual toggle writes to localStorage (via setStyleSentinel()),
one click permanently kills system-following.

Solution

Instead of returning early when a localStorage override is present, clear
the override and follow the system. The manual choice is still respected
across page loads, but it expires gracefully on the next OS theme change.

queryPrefersDark.addEventListener("change", e => {
  const alternate = e.matches;
  if (window.localStorage.getItem("quarto-color-scheme") !== null) {
    window.localStorage.removeItem("quarto-color-scheme");   // override expires
  }
  toggleColorMode(alternate);
  localAlternateSentinel = alternate ? 'alternate' : 'default';
  toggleGiscusIfUsed(alternate, darkModeDefault);
});

This is the only change. The existing sentinel machinery
(hasAlternateSentinel, getColorSchemeSentinel, setStyleSentinel) works
unchanged: when localStorage is absent, it falls back to
localAlternateSentinel (which always reflects the current effective mode).

Behavior

Scenario	Before	After
Page load, system dark → light	Follows	Follows
User toggles once	Override sticks forever	Override sticks until next OS theme change
User toggles, then system changes	Page ignores system	Override expires, page follows system
User toggles, navigates, reloads	Choice persists	Choice persists (until next OS change)

Files Changed

• src/resources/formats/html/templates/quarto-html-before-body.ejs
  (6 lines changed in the respectUserColorScheme event listener)

• tests/integration/playwright/tests/html-dark-mode-defaultdark.spec.ts
  (updated the '...do not respect-user-color-scheme after toggling' test to
  reflect the new behaviour: renamed to '...manual override expires on system theme change', assertions now expect the page to follow the system after a
  manual override expires, rather than sticking forever)

  All 14 existing tests in the dark-mode and light-mode suites continue to
  pass with these changes.

Fixes #14551

[posit-snyk-bot]

✅ Snyk checks have passed. No issues have been found so far.

Status	Scan Engine	Critical	High	Medium	Low	Total (0)
✅	Open Source Security	0	0	0	0	0 issues
✅	Licenses	0	0	0	0	0 issues

💻 Catch issues earlier using the plugins for VS Code, JetBrains IDEs, Visual Studio, and Eclipse.