[manifest] Remove background color on image object (#391)

Reached agreement with @mounirlamouri on IRC to remove this. We will figure out a different approach for splash screen.  @kenchris, you approve? 
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/manifest/pull/391

-- Commit Summary --

  * Remove background color on image object

-- File Changes --

    M index.html (53)

-- Patch Links --

https://github.com/w3c/manifest/pull/391.patch
https://github.com/w3c/manifest/pull/391.diff

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https://github.com/w3c/manifest/pull/391