Remove background color from operator shelves, instead hardcoding it per carrier (bug 1058163)

r? @ngokevin 

Will upstream to Fireplace after merge; all the relevant changes for Fireplace should be encapsulated in `feed.styl` and `feed_item.html` in this commit.
Yeh, don't know if that was in the style guide, but I'm sure shelves are an exception.