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.
It appends the parent selector, rather than appending it. So here it produces `[data-region=mx][data-carrier="telefonica"].feed-item.shelf .curve`