Is it possible to share formatting across margin boxes? I was trying to do something like:

@top-left-corner,@top-left,@top,@top-right,@top-right-corner,@bottom-left-corner,@bottom-left,@bottom,@bottom-right,@bottom-right-corner {
		background-color: #aaa;
		font-size: 90%;
		font-variant: small-caps;
		color: #161616;
	}

	@top-left-corner,@top-left,@top,@top-right,@top-right-corner {
		padding-bottom: 12px;
		vertical-align: bottom;
		border-bottom: 1px solid black;	
	}

	@bottom-left-corner,@bottom-left,@bottom,@bottom-right,@bottom-right-corner {
		padding-top: 12px;
		vertical-align: top;
		border-top: 1px solid black;	
	}

	@top-left,@bottom-left {
		text-align: left;
	}

	@top-right,@bottom-right {
		text-align: right;
	}

I've only been able to format one box at a time

No, we don't support this kind of syntax at the moment, sorry.