Now that we switched to using cover for provided-backgrounds, we should still be using contain for the fallback icon or else it will be stretched to fit the full preview size which isn't to spec. We should also revert back to the spec'd background-color, now that we are using background-size:cover for provided background images.
Attachment #8581833 - Flags: review?(florian) → review+
Verified fixed on Nightly 39.0a1 (2015-03-29) and Aurora 38.0a2 (2015-03-29), using Windows 7 (x64), Ubuntu 14.04 (x64) and Mac OS X 10.9.5.
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