css transparent background color

body {
  background-color: lightblue;
}
 

body {
  background-color: rgb(255,255,255);
}

.opacity30 {
  opacity: 0.3;
  filter: alpha(opacity=30); /* For IE8 and earlier */
}

html,body {
  background-color: red;
}

/* To apply color to background you have to use 'background-color' property and value of property is color name. */
html,body {
  background-color: blue;
}

div {
  width: 200px;
  height: 200px;
  display: block;
  position: relative;
}

div::after {
  content: "";
  background: url(image.jpg);
  opacity: 0.5;
  top: 0;
  left: 0;
  bottom: 0;
  right: 0;
  position: absolute;
  z-index: -1;   
}

div {
 opacity:25% ; 
}