transparent content area

I apologize in advance if this has been covered and I can’t find it. I have looked, but may have looked in the wrong place or wrong way. I am trying to make the main content area semi-transparent or transparent. I am thinking this is probably a CSS change, but I am still relatively new to this, and am not sure where to start. Thanks in advance for any help, or a nice direct “it’s right there, look harder next time” will be appreciated.