message: FormField: add plain mode styles & calendar header reset
diff: }
  /**
  - * ModalCardBase
  + * FormFieldMode.PLAIN
  */
  +.FormField--plain .FormField__border {
  + background-color: transparent;
  + border-color: transparent;
  +}
  +
  +.FormField--plain:focus-within .FormField__border,
  +.FormField--plain *:focus ~ .FormField__border,
  +.ChipsInput--focused .FormField--plain .FormField__border {
  + border-color: transparent;
  +}
  +/**
  + * CMP:
  + * ModalCardBase
  + */
  .ModalCardBase__header + .FormField,
  .ModalCardBase__subheader + .FormField {
  margin-top: 16px;
  border-top-right-radius: 0;
  }
  -.CustomSelect__open.Select--mimicry-plain.Select--mimicry:focus
  - .FormField__border,
  -.FormField:not(.FormField--disabled) .FormField__border--hover {
  - background-color: var(--background_page, var(--vkui--color_field_background));
  +/**
  + * CMP:
  + * CalendarHeader
  + */
  +.CalendarHeader__picker .FormField__after {
  + min-width: 24px;
  }
  