Hey! been on the forums a wile watching but heres me joined and posting!!
But anyway, to the problem, its no fault of dreamweaver, its due to the browser not supporting the property. If you run "Target Browser check' on the page dreamweaver will say something along the lines of 'CSS Property layer-background-color not supported' then a list of browsers. To keep the design you want try using a 1x1 pixel image in that color an set it as the layer background instead.