Firefox nightly 157.0a1: update renamed selectors

[github-actions]

Firefox nightly is now 157.0a1, and some selectors this
theme uses no longer match anything in it.

The changes here are the ones fxcss is certain about — a name that moved
between an id and a class. Anything it was less sure of is listed below
but not changed.

fxcss audit
  theme:   /Users/runner/work/WhiteSurFirefoxThemeMacOS/WhiteSurFirefoxThemeMacOS
  firefox: /Users/runner/work/_temp/Firefox.app/Contents/MacOS/firefox

  shipped chrome read from /Users/runner/work/_temp/Firefox.app/Contents/Resources/omni.ja, /Users/runner/work/_temp/Firefox.app/Contents/Resources/browser/omni.ja

  365 distinct id/class tokens in the theme
  collecting live elements:
    resting                1278 ids, 304 classes
    find bar open          1278 ids, 321 classes
    address bar results    1298 ids, 347 classes
    app menu opened        1343 ids, 364 classes
    context menus built    1378 ids, 375 classes
    dark mode              1378 ids, 375 classes

  2 selectors need attention

  RENAMED  #private-browsing-indicator-with-label  →  .private-browsing-indicator-with-label
           same name, now a class rather than an id

    chrome/WhiteSur/parts/headerbar-private-urlbar.css:42
    - #TabsToolbar #private-browsing-indicator-with-label {
    + #TabsToolbar .private-browsing-indicator-with-label {

  SIMILAR  #placesToolbar  →  #PlacesToolbar
           no exact match; closest live name is #PlacesToolbar

    chrome/WhiteSur/parts/places.css:116
    - #placesToolbar {
    + #PlacesToolbar {

    chrome/WhiteSur/parts/places.css:122
    - #placesToolbar toolbarbutton,
    + #PlacesToolbar toolbarbutton,

    chrome/WhiteSur/parts/places.css:127
    - #placesToolbar toolbarbutton:not([disabled]):hover,
    + #PlacesToolbar toolbarbutton:not([disabled]):hover,

  78 token(s) live in states this audit could not open — the
  shipped chrome still carries them (dialogs, other platforms), so they
  are healthy and not counted above.

  115 other token(s) were not seen in any state fxcss could
  produce. That usually means a platform-specific or state-specific
  element rather than a removed one, so they are not counted above.
  Pass --all to list them.

  wrote fxcss-fix.patch (1 file(s)); review it, then:
    git apply fxcss-fix.patch

  --strict: failing because 2 selector(s) need attention

Worth previewing before merging: the PR preview workflow will render
before and after on this pull request.