You would use Css to make the style different, if thats what you mean.
Here is a FULL example:
PAGE.html
<html>
<head><tite>Your webpage Title</title>
<link rel="stylesheet" type="text/css" href="STYLE.css" />
</head>
<body>
<form action = "POST.php" method = "POST">
<input type = "text" name = "username">
</form>
</body>
</html>
STYLE.css
input {
background-color: #666699;
}
POST.php
<?php
$user = $_POST['user'];
if ($user==john)
echo "Welcome John";
else
echo "Go away $user";
?>