background: rgba(0, 0, 0, 0.5); 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 */