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 was previously alpha-ed, wasn't it?

https://github.com/mozilla/transonic/pull/71/files#diff-b0e38e7cc8d44bac6d018fb4749068d0L209