Change default font-family

Hello,

First you should know that a command that allows you to modify ALL text in a single movement is not really possible.
You will therefore have to add several selectors to be sure to cover everything, for example:

a, h3, body {
    font-family: arial !important;
}

Hello,

There is no options, settings or else to modify in HikaShop only the default font.
If your template has a command to define a font-family but it does not reach the HikaShop pages, then this template is not HikaShop compatible.
And so my answer is still good to get what you need, plus we haven't any solution to suggest.
And for you can use this kind of command to remove "italic" :

a, h3, body {
    font-family: arial !important;
    font-style: norma !important;
}

Hello,

First a little tricks, go to your "Hikashop Cart notification" Plugin, and define an high "Auto-hide delay" in the relative option, and with no "Redirect to checkout".

This will allow you, to keep your popup displayed, then you will then have plenty of time to use your browser inspector tool ( Documentation ) to see your html and required class to build your selector.

Regards