Bug 311727 – Implement `color_scheme_dark` manifest member

• Component: WebKit Misc.
• Status: NEW

[Marcos Caceres]

The Web App Manifest spec (w3c/manifest PR #1207) adds a `color_scheme_dark` member that allows web apps to specify alternative `theme_color` and `background_color` values for dark-mode operating system themes.

**Spec PR:** https://github.com/w3c/manifest/pull/1207
**Closes manifest issue:** https://github.com/w3c/manifest/issues/975

## What to implement

`color_scheme_dark` is an ordered map whose keys are "themeable members" (`theme_color`, `background_color`) and whose values are CSS color strings.

When applying a manifest and the OS uses a dark color theme, for each themeable member that exists in `color_scheme_dark`, the UA SHOULD use the dark value in place of the base value, unless user accessibility preferences take precedence.

**Example manifest:**
```json
{
  "background_color": "#fff",
  "theme_color": "red",
  "color_scheme_dark": {
    "background_color": "#000",
    "theme_color": "hotpink"
  }
}
```

## Implementation notes

- Parse `color_scheme_dark` as an ordered map during manifest processing (see `process the color_scheme_dark member` algorithm in the spec)
- Iterate over the fixed list « "theme_color", "background_color" » (not arbitrary keys)
- Use the dark values when the system is in dark appearance (UIUserInterfaceStyleDark / NSAppearance.currentAppearance)
- Existing `theme_color` and `background_color` behavior is unchanged when `color_scheme_dark` is absent or the system is in light mode

## Status

Spec PR is open; waiting for at least one implementation commitment before merging. This bug is the WebKit commitment.

[Radar WebKit Bug Importer]

<rdar://problem/174315709>

[Daniel Murphy]

Hello!

Chromium intends on supporting this as well, this is our implementation tracking bug: https://crbug.com/383165202

Dan

[Devin Rousso]

Pull request: https://github.com/WebKit/WebKit/pull/62443