Finding the conditional formatting in the card visual is a bit tricky. There is no separate option for that. You need to go to the Format tab of the visual, and then expand the Data Label. The right beside the Data Label's colour you need to hover your mouse, and you will find a three dots icon appearing, which if you click on it, you will see Conditional Formatting.

Now in the Conditional Formatting tab, you can apply it in different methods. for example, you can choose Rules, and then

The Rules mode will give you the ability to put custom roles as below;

Reference:
https://radacad.com/enhance-the-card-visual-in-power-bi-with-conditional-formatting