Check out example codes for "css how to make background transparent". It will help you in understanding the concepts better.

Code Example 1

.img1 {
opacity: 0.6;
z-index=1;
}

Code Example 2

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

Code Example 3

/* Use RGBA */
background-color: rgba(255, 255, 0, 0.75); /* Transparent Yellow */

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