This allows you to style a/some elements on the web page (MagicMirror page is presented in a browser)

This is a huge topic area, I looked at the anylist module provided css, and it doesn’t provide any direct styling on the text elements

So you will have to discover what they are. See the second link in my signature below for a short intro to the browser developers window, elements tab

The content for CSS styles is HUGE
I would use your favorite search engine on how to set an element style the way you want, this will at least give you the names of the styles you want to set. Then the example will teach you how to find the element you want to style

Watch out for opaque, if you later decide on pictures in the background, you will have a black rectangle showing.