http://stackoverflow.com/questions/637921/opacity-of-background-but-not-the-text

CSS background transparency without affecting child elements, through RGBa and filters

[css]
.alpha60 {
/* 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)”;
}
[/css]

StartColorStr
　　　　　設定値は#00000000～#ffffffff
　　　　　設定値は８桁表示で#aarrggbb
　　　　　aa＝alpha　00：透明～ff：不透明
　　　　　不透明なら６桁指定も可