Custom css not working

I’m trying to add custom css to my Dash application (by defining css files inside assets folder) but it does not seem to be working. I have tried the example provided in Adding Your Own CSS and JavaScript to Dash Apps Section in the documentation but the css classes are not producing any effect. My Dash version is 1.8.0. Thanks

It suddenly started to work in my main app and I have no idea what I have done! It still does not seem to work in the example mentioned above though…