I have not any screen for this, but I just want to modify the input box html from that :
<input class="inputbox" id="address_post_code" onchange="hikashopToggleFields(this.value,'address_post_code','address',0);" type="text" name="data[address][address_post_code]" value="">
to that :
<input class="b-form__input--small" id="address_post_code" onchange="hikashopToggleFields(this.value,'address_post_code','address',0);" type="text" name="data[address][address_post_code]" value="" placeholder="Code postal *">