Hi,
For the quantity input, you can change the "Quantity input method" setting in the HikaShop configuration.
Regarding the font used on the price, it depends on the CSS of your template. You can add extra CSS code to change the font and have the prices look however you want. I'm not sure what you mean by "The other thing that bothers me is that the custom.css styling for the font of the price doesn't work" as there is no custom.css file in HikaShop. I would need more information (what CSS code was added where ? and the URL of the page ) to check on the situation and tell you what you're missing.