You really need to read the scripting guide.
Application is a Photoshop Class. To reference that class you use app. So the first line in the function should be app.activeDocument. Even then that line does not do anything except return a document object. If you do not store the returned object in a variable you might as well delete that line.
app.backgroundColor is an app property, not a method. So the () are not used. That property either returns a SolidColor object that is the current background color or you can assign it a new color something like this
var myColor = new SolidColor();
myColor.rgb.red = 1;
myColor.rgb.green = 12;
myColor.rgb.blue - 34;
app.backgroundColor = myColor;
But that does not create a new layer and I am not sure what kind of layer you want. Do you want to create a new artLayer then fill it with your color or do you want to create a new SolidFill adjustment layer using that color.
To create a new artLayer you do something like
app.activeDocument.artLayers.add();
To create a new adjustment layer you would need to use Action Manager( scriptlistener ).