templates/base.html

<!doctype html>
<html lang="en">
<head>
  <meta charset="utf-8" />
  <meta name="viewport" content="width=device-width, initial-scale=1" />
  <title>{{ title if title else "Flask Portfolio" }}</title>
  <link href="https://cdn.jsdelivr.net/npm/bootstrap@5.3.3/dist/css/bootstrap.min.css" rel="stylesheet">
  <link href="https://cdn.jsdelivr.net/npm/bootstrap-icons@1.11.3/font/bootstrap-icons.css" rel="stylesheet">
  <link href="{{ url_for('static', filename='style.css') }}" rel="stylesheet">
</head>
<body>
  {% block content %}{% endblock %}
</body>
</html>