Most of the time, card-mod won’t cause any errors: the styles simply won’t be applied. You will need to inspect and manipulate the DOM by hand in your browser to try and figure out what is wrong. I’m running 2022.2.9 with card-mod 3.1.2. Maybe it’s something with your versions.
Try to seek help or find more examples here:
It’s defined by the default theme, so it would depend on the theme you use how it’s set. You can see this using the developer tools in your browser.