How to change the alert style of js, such as background color _ javascript skills

How to change the alert style of js, for example, the background color and background transparency in the pop-up dialog box. Below are some examples for interested users to learn. The Code is as follows:

Window. alert = function (str)
{
Var shield = document. createElement ("DIV ");
Shield. id = "shield ";
Shield. style. position = "absolute ";
Shield. style. left = "0px ";
Shield. style. top = "0px ";
Shield. style. width = "100% ";
Shield. style. height = document. body. scrollHeight + "px ";
// The background color in the pop-up dialog box
Shield. style. background = "# fff ";
Shield. style. textAlign = "center ";
Shield. style. zIndex = "25 ";
// Valid for background transparent IE
// Shield. style. filter = "alpha (opacity = 0 )";
Var alertFram = document. createElement ("DIV ");
AlertFram. id = "alertFram ";
AlertFram. style. position = "absolute ";
AlertFram. style. left = "50% ";
AlertFram. style. top = "50% ";
AlertFram. style. marginLeft = "-225px ";
AlertFram. style. marginTop = "-75px ";
AlertFram. style. width = "pixel PX ";
AlertFram. style. height = "150px ";
AlertFram. style. background = "# ff0000 ";
AlertFram. style. textAlign = "center ";
AlertFram. style. lineHeight = "150px ";
AlertFram. style. zIndex = "300 ";
StrHtml ="

\ N ";
StrHtml + ="
• [Custom prompt]
\ N ";
StrHtml + ="
• "+ Str +"
\ N ";
StrHtml + ="
\ N ";
StrHtml + ="

\ N ";
AlertFram. innerHTML = strHtml;
Document. body. appendChild (alertFram );
Document. body. appendChild (shield );
Var ad = setInterval ("doAlpha ()", 5 );
This. doOk = function (){
AlertFram. style. display = "none ";
Shield. style. display = "none ";
}
AlertFram. focus ();
Document. body. onselectstart = function () {return false ;};
}

Effect