I have to edit add to cart button on product page for it to look like images below (same color, font size, black text outline and different size on desktop and mobile (responsive)). If it’s of any significant the size of “add to cart” button image (the green one) on mobile is full width (100%) while on desktop it’s around 56%.
How it should look on desktop:
How it should look on mobile (responsive full width):
Those changes require lots of code customization, on which we cannot help you out from this forum. If you really want to make those changes, you can contact our customization team. https://themegrill.com/customization/