Hello there!
The Shopify Polaris team took a design decision where component customisation is generally not possible, allowed or even endorsed because it isn't meant to be a general purpose UI component library and they'd like to avoid cases where app developers go off on a tangent. They have many good reasons why they chose to do so as can be read here if you're interested.
For you specific use case, you could simply have a nested element in the <Card /> like shown here in my quick Pen (check App.js. Used styled-components, works with any of the React styling methods).
However, just because it is possible, I wouldn't recommend it and would proceed carefully as the last thing you want to do is introduce inconsistency within the app UI.
Best wishes!
