Form related classes.
Starterkit form.css
Default theme css.
/core/themes/starterkit_theme/css/components/form.css
.form-item.form-wrapper.form-actions
.label.description
.form-composite # Used in radios, checkboxes - see CompositeFormElementTrait
.fieldset-wrapper
.form-checkboxes.form-radios.form-type-checkbox.form-type-radio