Add color_scheme_dark member and themeable members

[christianliebel]

Closes #975

This change (choose at least one, delete ones that don't apply):

• Adds new normative requirements

Implementation commitment (delete if not making normative changes):

• WebKit (https://bugs.webkit.org)
• Chromium (https://bugs.chromium.org/)
• Gecko (http://bugzilla.mozilla.org)

If change is normative, and it adds or changes a member:

• updated JSON schema

Commit message:

Add color_scheme_dark member and themeable members

Person merging, please make sure that commits are squashed with one of the following as a commit message prefix:

• chore:
• editorial:
• BREAKING CHANGE:
• And use none if it's a normative change

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Preview | Diff

[Copilot]

Pull request overview

Adds support for defining dark-mode theme overrides in the Web App Manifest by introducing a color_scheme_dark member and identifying which members are “themeable” (and thus eligible for dark-mode overrides).

Changes:

• Marks icons, theme_color, and background_color (and shortcut icons) as “themeable members”.
• Defines manifest/color_scheme_dark and shortcut item/color_scheme_dark, including processing steps.
• Hooks Process the \color_scheme_dark` member` into the main manifest processing algorithm.

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[Copilot]

The color_scheme_dark definition says its keys can be themeable members and their corresponding localizable members (e.g., icons_localized), but the “Theming themeable members” paragraph says the map accepts only themeable members as keys. Please align the wording so it consistently includes the *_localized companion keys (or redefine them as themeable) to avoid ambiguity about what keys are allowed.

Suggested change

	[=themeable members=] as keys, and their values as the theme
	overrides.
	[=themeable members=] and their corresponding [=localizable members=]
	as keys, with their values as the theme overrides.

[Copilot]

themeable member is defined as “a manifest member that can be themed”, but the term is also used for shortcut item/icons, which is not a top-level manifest member. Please broaden the definition so it applies to members of nested manifest structures (e.g., shortcut items) as well, or avoid using the same term for shortcut-item members.

Suggested change

	A <dfn>themeable member</dfn> is a [=manifest=] member that can be
	A <dfn>themeable member</dfn> is a member of the [=manifest=] or of
	a nested manifest structure (such as a shortcut item) that can be

[Copilot]

Process image resources always sets map[member] to a new (possibly empty) list before validating images (see Processing image resources at index.html:2364-2370). Calling it unconditionally here means that if color_scheme_dark.icons is absent, the processed dark-scheme override will still contain an empty icons list, which prevents inheriting the main icons and can effectively remove icons in dark mode. Consider only invoking Process image resources when "icons" exists in |colorScheme| (and is a list), so absence cleanly falls back to the light-scheme icons.

Suggested change

	<li>[=Process image resources=] passing |colorScheme|["icons"],
	<li>If |colorScheme|["icons"] [=map/exist=] and is a [=list=], then
	[=Process image resources=] passing |colorScheme|["icons"],

[Copilot]

Same inheritance issue for shortcut-item theming: Process image resources always sets processedColorScheme["icons"] to an empty list even when color_scheme_dark.icons is missing/invalid (index.html:2364-2370). This makes a present-but-partial shortcut item/color_scheme_dark override wipe out the base shortcut icons. Add a presence/type check for |colorScheme|["icons"] before processing so missing icons inherits from the light-scheme shortcut icons.

Suggested change

	<li>[=Process image resources=] passing |colorScheme|["icons"],
	|processedColorScheme|, |manifest URL|, and "icons".
	<li>If "icons" [=map/exists=] in |colorScheme| and
	|colorScheme|["icons"] is a [=list=], then [=Process image
	resources=] passing |colorScheme|["icons"], |processedColorScheme|,
	|manifest URL|, and "icons".

[christianliebel]

.cc @carlosjeurissen