Add css only in one product page

Hi,

Yes, it's possible.
If you look at the HTML of the area, you can see that there is a parent div which has a unique id for each product of the listing.
For example:
monosnap.com/file/Iv9asKU6PGPxHP4vUPnE1DiFjOoig6
So for that product of my screenshot, you can do:
#div_hikashop_category_information_module_140_3063 .hikacart{ background-color: red !important; }
And the background color of the add to cart button will be red for that particular product with the id 3063 on the products listing of the module with the id 140.