background: rgba(225, 225, 225, .5);
This ^^.The opacity is used to change the opacity of a division or whatever you're using. To change only the background color, you would need to use an rgba. If you're using color white, then type:
HTML:background: rgba(225, 225, 225, .5);
The .5 is the opacity. Play with it but it only goes up to .99. Hope this answered your question.