Read the docs and follow the code. It’s explained there why you need mod_card (which is part of card_mod, but for advanced usages like styling cards without ha-element, like grid, vertical etc). And how to use the code line.
Sidenote: this is meant to style individual cards, not entire themes. So getting kina off-topic since this thread is for card mod for themes. I suggest moving over to this topic if you need more help with styling cards outside of themes: 🔹 Card-mod - Add css styles to any lovelace card