background-color: rgba(0, 0, 0, 0.2); code example
Example 1: background color cs rgba
#p1 {background-color:rgba(255,0,0,0.3);}
Example 2: background color opacity
rgba(51, 170, 51, .1) /* 10% opaque green */
rgba(51, 170, 51, .4) /* 40% opaque green */
rgba(51, 170, 51, .7) /* 70% opaque green */
rgba(51, 170, 51, 1) /* full opaque green */