Mushroom Cards - Build a beautiful dashboard easily πŸ„ (Part 2)

How to integrate this into yaml THEME in order to be applied to each card. This is from the guide in the first post Mushroom Cards Card Mod Styling/Config Guide - #10 by dimitri.landerloos

   mushroom-shape-icon$: |
     .shape {
       background-color: transparent !important;
     }

?

This construction:

    ha-card.type-custom-mushroom-light-card 
    {
      mushroom-shape-icon$: |
        .shape {
          background-color: transparent !important;
        }
    }

doesn’t work