Returns a style object (specifically, a CSSStyleDeclaration object in the W3C DOM terminology) showing the net cascade of style settings that affect the element passed as the first parameter. To retrieve a particular style attribute value (including a value inherited from the default browser style sheet), use the getPropertyValue( ) method of the style object returned from this method:

var compStyle = getComputedStyle(document.getElementById("myP"), "");
var pBGColor = compStyle.getPropertyValue("background-color");