It’s not necessary to import a module. To find out more about adding css to your dashboard see: Adding CSS & JS and Overriding the Page-Load Template | Dash for Python Documentation | Plotly
If you are not using Bootstrap, a good place to start is the basic stylesheet defined here: https://codepen.io/chriddyp/pen/bWLwgP.css
You can add it to your app as an exernal stysheet like this:
external_stylesheets = ['https://codepen.io/chriddyp/pen/bWLwgP.css']
app = Dash(__name__, external_stylesheets=external_stylesheets)
Or download it, modify it as you like, and add it to .css file in the assets/ folder.