[IMP] theme_bootswatch: add css for display qty field in shop (flatly or cosmo theme)

<b>Fix this</b> :  Cosmo and Flatly both themes having a same problem product quantity does not appear on that input box.

<b>Pad</b> : https://pad.odoo.com/p/r.hoWDS8Ud6QRvZS2k
<b>Task</b> : https://www.odoo.com/web#id=10747&view_type=form&model=project.task&menu_id=3942&action=327