Doing this is only possible with Unity Pro, but if you do have Unity Pro (I don't) I think you need to put this part:

var params = {
       backgroundcolor: "A0A0A0", // This part changes the backround color
       bordercolor: "000000", // This part changes the border color
       textcolor: "FFFFFF", // This part changes the color of some text. 
       logoimage: "MyLogo.png", // What splash logo do you want?
       progressbarimage: "MyProgressBar.png", // What loading bar do you want?
       progressframeimage: "MyProgressFrame.png" // What loading bar border do you want?
};

   unityObject.embedUnity("unityPlayer", "WebPlayer.unity3d", 600, 450, params); 

In between in your HTML file.

In unityObject.embedUnity(), I'm not sure what the "unityPlayer" thing is, but "WebPlayer.unity3d" should be replaced with the relative pathname to your unity3d file, 600, 450 are the width/height.

Edit: Actually, "unityPlayer" is a way for the browser to know which object has the player in it. Credit to the commentator below.

you need to put this vars in the hmtl generated by unity when you press build&run for example webplayer.html with this code is the part code of the webplayer.html. (u can see it and know what u need to change.

sorry for my english!!

    function GetUnity() {
        if (typeof unityObject != "undefined") {
            return unityObject.getObjectById("unityPlayer");
        }
        return null;
    }
    if (typeof unityObject != "undefined") {
        var params = {
                          **place to put your code!!!**

        };
        unityObject.embedUnity("unityPlayer", "WebPlayer.unity3d", 600, 512, params);
    }
    -->

Can someone Help me...???