<form method="POST" action="index.php">
Username: <input type="text" class="pole" name="username" value="<?=$_POST['username']?>" /><br />
Password: <input type="password" class="pole" name="password" /><br />
<input name="login" type="submit" value="Login!" />
</form>
Добавяш това...
И после добавяш във html файла
<head>
...
...
...
<style type="text/css">
input.pole {
background-color: #цвят;
}
</style>
</head>
Точките където са ти другите неща между хеад..
Иначе можеш и така да го направиш..
<form method="POST" action="index.php">
Username: <input type="text" name="username" style="background-color: #fff;" value="<?=$_POST['username']?>" /><br />
Password: <input type="password" name="password" style="background-color: #fff;" /><br />
<input name="login" type="submit" value="Login!" />
</form>