Make post backgrounds transparent

I’ve read through all of the posts relating to transparent backgrounds and haven’t found the answer yet. I’m getting closer I think, but…
I can’t determine if the answer is CSS or changing the code of each post background to transparent, in either case I’m having a hard time locating the post box’s that need to be modified. i’ve read that using RGBa would be good for this, but how and where do I locate the post background to add the RGBa… and… how to add the RGBa to make the post background transparent.
I have already applied this code (below) in the theme options Custom CSS and it works great for the main background, but the posts still have a white box I want to make transparent also. Any ideas, tutorial?? thanks for any help.
#wrapper,
#featured {
-moz-border-radius: 0;
-webkit-border-radius: 0;
background-color: transparent;
border: none;
border-radius: 0;