Hi,
No, I'm talking about the setting below the "frontend CSS file".
The problem is that you removed all the style files and thus you don't see the dropdown with the styles selection and thus can't select none in it.
Change the line:
return '<a target="_blank" href="'.HIKASHOP_REDIRECT.'hikashop-styles'.'">'.hikashop_tooltip(JText::_('STYLE_TOOLTIP_TEXT'), JText::_('STYLE_TOOLTIP_TITLE'), '', JText::_('STYLE_HIKASHOP')).'</a>';
to:
return '<input type="hidden" value="" name="'.$map.'"/><a target="_blank" href="'.HIKASHOP_REDIRECT.'hikashop-styles'.'">'.hikashop_tooltip(JText::_('STYLE_TOOLTIP_TEXT'), JText::_('STYLE_TOOLTIP_TITLE'), '', JText::_('STYLE_HIKASHOP')).'</a>';
in the file administrator/components/com_hikashop/types/css.php and then save the configuration of HikaShop. That should solve the problem automatically.