Sorry if there was any confusion here. What I am looking to do is change the opacity of the the black background that the modal window sits on top of. So I'd want to see more of the page in the background and not necessary change the opacity of the modal window itself.
RGBA stands for "Red, green, blue, alpha", in web, the alpha will be your opacity level. So on this code, changing your '.18' to a higher number will make it more opaque, and a lower number will make it more transparent.
You mentioned using the style of 'opacity', while this could do it, opacity behaves a little differently. Opacity will change the entire container (contents and all), so everything in that container will be come transparent/luscent.