Using stylesheets, images and scripts will improve the appearance and experience of your app greatly.
Place these files inside a folder named: static. Then simply reference the files’ locations in your templates

The app directory should look something like this:

my_app.py
[templates]
   |__ add.html
   |__ index.html
   |__ layout.html
[static]
 |__ scripts.js
 |__ styles.css

App Files Update

styles.css

body {
 background: beige;
 color: #333;
}

scripts.js

alert("Welcome!");

layout.html

...
  <link href="/static/styles.css" type="text/css" rel="stylesheet">
...
  {% block scripts %} {% endblock %}
  </body>
</html>

index.html

{% block scripts %}
<script src="/static/scripts.js"></script>
{% endblock %}