In order to add custom CSS to the theme:

Go to admin > Online store > Themes > Customizer > Settings > Custom CSS.

• Custom option label texts

#expo-section .expo-element-label {
  font-size: ...;
  font-weight: ...;
}

• Custom choice list value/ checkbox value text style

#expo-section .expo-choice-list__option,
#expo-section .expo-choice-list__option {
  font-size: ...;
  font-weight: ...;
  ...
}

• Hide extra cost text

#expo-section .expo-option-extra-price {
  display: none;
}

• Change the Button option radius

.expo-buttons__option-label {
  border-radius: 15px;
}