[manifest] Add background_color member. (#396)

@kenchris @marcoscaceres please take a look.
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/manifest/pull/396

-- Commit Summary --

  * Add background_color member.

-- File Changes --

    M index.html (69)

-- Patch Links --

https://github.com/w3c/manifest/pull/396.patch
https://github.com/w3c/manifest/pull/396.diff

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https://github.com/w3c/manifest/pull/396