If you set opacity on an element it will affect all its child elements. To set an opacity just on the background of an element you will have to use RGBA colors. Following example will have a black background with 0.6 opacity.

/* Fallback for web browsers that don't support RGBa */
background-color: rgb(0, 0, 0);

/* RGBa with 0.6 opacity */
background-color: rgba(0, 0, 0, 0.6);

/* For IE 5.5 - 7*/
filter: progid:DXImageTransform.Microsoft.gradient(startColorstr=#99000000, endColorstr=#99000000);

/* For IE 8*/
-ms-filter: "progid:DXImageTransform.Microsoft.gradient(startColorstr=#99000000, endColorstr=#99000000)";