Adding css to dash app

I have been exploring styling the dash app further using css stylesheet as explained in the documentation Adding CSS & JS and Overriding the Page-Load Template | Dash for Python Documentation | Plotly.

However I am struggling to convert the html css code to my dash app.

For example: I have a mullti select dropdown and I wish to change the color of the menu select, I can see the relevant html css code when inspecting the element, however I do not know how to convert it to the relevant syntax in my assets folder.

This is a mouthful, please see screenshots.

Dropdown with white background and text. (This white background and text is what I would like to change to make the options visible)
This is the code retrieved from inspect element where I can change to the desired result
The code that it needs to be converted to in my assets folder.

Hope this makes sense.