Twenty Thirteen: audio and video post formats have same background-color

[MikeHansenMe]

When you have an audio post then a video post they appear to blend because of the same background-color. We should change one of the post formats to another color to create the separation. See Screenshots

Before

After

Patch changing the bg using color from header image

There are other pairs that have the same color scheme (Link and Aside; Standard and Image). I don't see why audio and video should be considered a particularly special case, nor do I see much benefit in creating multiple new color schemes.

Discussed a bit more in office hours, IRC today ​log and decided to go with original design intent and color scheme set up by Joen.

See also #23818

#23818 has a proposed solution from Joen.

In [23785] Twenty Thirteen: improve separation between Link and Aside posts, props Joen for starter CSS rules. Fixes #23818 and closes #23707 as related.