Reducing opacity only on background post and not content

Hello again,
I found this code very helpful and as you can see I added
opacity: 0.5;

/*Change background of headings */
#primary.content-area {
background:#3f3f3f; opacity: 0.5;
}
Now that I am adding 0.5 of opacity ( the intention is to reduce only background opacity)
my whole post is 0.5 of opacity

Is there a way to reduce only background opacity and not the text and content fo the post?
you can see it here
http://jeromemeyer.com/?p=44
thanks for your feedback!