checkout-form.css

/* Block component */
.checkout-form {
  // ..
}
/* Element that depends upon the block */
.checkout-form__title {
  // ..
}
.checkout-form__input {
  // ..
}
.checkout-form__button {
  // ..
}
/* Modifier that changes the style of the block */
.checkout-form--darkmode {
 // ...
}
/* Modifier that changes the style of the element */
.checkout-form__input--big {
 // ...
}