Puoi fare senza i css...
codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>Untitled</title>
</head>
<body bgcolor="blue">
<input type="button" value="Cambia colore" onclick="document.body.bgColor='red'">
</body>
</html>
o con ....
codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>Untitled</title>
</head>
<body style="background-color:blue">
<input type="button" value="Cambia colore" onclick="document.body.style.backgroundColor='red'">
</body>
</html>