оно??
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN" "http://www.w3.org/TR/html4/frameset.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
</head>
<script type="text/javascript">
var tag_css = document.createElement('style');
tag_css.type = 'text/css';
tag_css.innerHTML = '.modal_content { width: 500px; position: absolute; z-index: 10; left: 50%; top: 200px; margin: -150px 0 0 -260px; border: 1px solid black;}'
var tag_head = document.getElementsByTagName('head');
tag_head[0].appendChild(tag_css);
</script>
<body>
<div class="modal_content"></div>
</body>
</html>