A basic HTML file should look like this:
<!doctype html>
<html lang="en">
<head>
<!-- Required meta tags -->
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<!-- The page supports both light and dark color schemes, with light being default -->
<meta name="color-scheme" content="light dark">
<!-- Bootstrap CSS (as per normal) -->
<link href="https://cdn.jsdelivr.net/npm/[email protected]/dist/css/bootstrap.min.css" rel="stylesheet">
<!-- Add the Bootstrap-Nightfall Variant CSS (the media attribute is for dark auto-switching) -->
<link href="https://cdn.jsdelivr.net/npm/[email protected]/dist/css/bootstrap-nightfall.min.css" rel="stylesheet" media="(prefers-color-scheme: dark)">
<title>Hello, world!</title>
<!-- Optional Meta Theme Color is also supported on Safari and Chrome -->
<meta name="theme-color" content="#111111" media="(prefers-color-scheme: light)">
<meta name="theme-color" content="#eeeeee" media="(prefers-color-scheme: dark)">
</head>
<body>
<!-- ////////////// -->
<!-- Your HTML here -->
<!-- \\\\\\\\\\\\\\ -->
<!-- Optional Bootstrap JavaScript -->
<script src="location/of/the/bootstrap.js/here"></script>
</body>
</html>
Refer to the Bootstrap 5 Docs for more help.