styled radio buttons

HTML

<script src="//netdna.bootstrapcdn.com/bootstrap/3.1.1/js/bootstrap.min.js"></script>
<link rel="stylesheet" href="//netdna.bootstrapcdn.com/bootstrap/3.1.1/css/bootstrap-theme.min.css">
<link rel="stylesheet" href="//netdna.bootstrapcdn.com/bootstrap/3.1.1/css/bootstrap.min.css">
<form class="form-signin" action="firstLogin.action" method="post">
     <h2 class="form-signin-heading">Please sign in</h2>

    <input type="text" class="form-control" name="username" placeholder="User Name" required autofocus>
    <br>
    <input type="password" class="form-control" name="password" required placeholder="Password">
    <br>
    <div data-toggle="buttons">
        <div class="btn-group">
            <label class="btn btn-primary"> 
            <input type="radio" name="type" id="type" value="admin" class="sr-only" required>Admin</label>
            <label class="btn btn-primary">
                <input type="radio" name="type" id="type" value="user" class="sr-only" required>User</label>
        </div>
    </div>
    </div>
    <br>
    <button class="btn btn-lg btn-primary btn-block" type="submit">Sign in</button>
</form>