Live Demo: Bootstrap Flat Modal Login Modal Form

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">
<title>Bootstrap Flat Modal Login Modal Form</title>
<link rel="stylesheet" href="https://fonts.googleapis.com/css?family=Roboto|Varela+Round">
<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.5.0/css/bootstrap.min.css">
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css">
<script src="https://code.jquery.com/jquery-3.5.1.min.js"></script>
<script src="https://cdn.jsdelivr.net/npm/popper.js@1.16.0/dist/umd/popper.min.js"></script>
<script src="https://stackpath.bootstrapcdn.com/bootstrap/4.5.0/js/bootstrap.min.js"></script>
<style>
body {
	font-family: 'Varela Round', sans-serif;
}
.modal-login {
	width: 350px;
}
.modal-login .modal-content {
	padding: 20px;
	border-radius: 5px;
	border: none;
}
.modal-login .modal-header {
	border-bottom: none;
	position: relative;
	justify-content: center;
}
.modal-login .close {
	position: absolute;
	top: -10px;
	right: -10px;
}
.modal-login h4 {
	color: #636363;
	text-align: center;
	font-size: 26px;
	margin-top: 0;
}
.modal-login .modal-content {
	color: #999;
	border-radius: 1px;
	margin-bottom: 15px;
	background: #fff;
	border: 1px solid #f3f3f3;
	box-shadow: 0px 2px 2px rgba(0, 0, 0, 0.3);
	padding: 25px;
}
.modal-login .form-group {
	margin-bottom: 20px;
}
.modal-login label {
	font-weight: normal;
	font-size: 13px;
}
.modal-login .form-control {
	min-height: 38px;
	padding-left: 5px;
	box-shadow: none !important;
	border-width: 0 0 1px 0;
	border-radius: 0;
}
.modal-login .form-control:focus {
	border-color: #ccc;
}
.modal-login .input-group-addon {
	max-width: 42px;
	text-align: center;
	background: none;
	border-bottom: 1px solid #ced4da;
	padding-right: 5px;
	border-radius: 0;
}
.modal-login .btn, .modal-login .btn:active {        
	font-size: 16px;
	font-weight: bold;
	background: #19aa8d !important;
	border-radius: 3px;
	border: none;
	min-width: 140px;
}
.modal-login .btn:hover, .modal-login .btn:focus {
	background: #179b81 !important;
}
.modal-login .hint-text {
	text-align: center;
	padding-top: 5px;
	font-size: 13px;
}
.modal-login .modal-footer {
	color: #999;
	border-color: #dee4e7;
	text-align: center;
	margin: 0 -25px -25px;
	font-size: 13px;
	justify-content: center;
}
.modal-login a {
	color: #fff;
	text-decoration: underline;
}
.modal-login a:hover {
	text-decoration: none;
}
.modal-login a {
	color: #19aa8d;
	text-decoration: none;
}	
.modal-login a:hover {
	text-decoration: underline;
}
.modal-login .fa {
	font-size: 21px;
	position: relative;
	top: 6px;
}
.trigger-btn {
	display: inline-block;
	margin: 100px auto;
}
</style>
</head>
<body>
<div class="text-center">
	
	<a href="#myModal" class="trigger-btn" data-toggle="modal">Click to Open Login Modal</a>
</div>

<div id="myModal" class="modal fade">
	<div class="modal-dialog modal-login">
		<div class="modal-content">
			<div class="modal-header">				
				<h4 class="modal-title">Sign In</h4>
                <button type="button" class="close" data-dismiss="modal" aria-hidden="true">&times;</button>
			</div>
			<div class="modal-body">
				<form action="/examples/actions/confirmation.php" method="post">
					<div class="form-group">
						<div class="input-group">
							<span class="input-group-addon"><i class="fa fa-user"></i></span>
							<input type="text" class="form-control" name="username" placeholder="Username" required="required">
						</div>
					</div>
					<div class="form-group">
						<div class="input-group">
							<span class="input-group-addon"><i class="fa fa-lock"></i></span>
							<input type="text" class="form-control" name="password" placeholder="Password" required="required">
						</div>
					</div>
					<div class="form-group">
						<button type="submit" class="btn btn-primary btn-block btn-lg">Sign In</button>
					</div>
					<p class="hint-text"><a href="#">Forgot Password?</a></p>
				</form>
			</div>
			<div class="modal-footer">Don't have an account? <a href="#">Create one</a></div>
		</div>
	</div>
</div>     
</body>
</html>