Ah, got it. At least I'm pretty sure.
Both input.art-button and .art-button have CSS applied to them in your template CSS, incl font-size and padding. Guess that's what rules, so you won't see changes when messing with .hikashop_cart_input_button in the HikaShop CSS file.
So, change what I gave you before to
#hikashop_product_contact_main .art-button {
font-size: 20px !important;
padding: 18px 20px !important;
}
... of course, with values adjusted to your liking. You don't need to add more classes as the ID hikashop_product_contact_main is used only there.
You seem to have no custom or override CSS for your template, so then leave it in your HikaShop custom frontend file, where you said you placed it already.
If THAT doesn't work, I shall retire... in hell...