Hello,
As far as we have visited your Site and confirmed, the Add to Cart button style right now is appearing the same on your Site as of the Megamarket Demo: https://ibb.co/T23kzM9
Further, to configure the Buttons under the Theme you can go to Dashboard > Theme Settings > Styles and colors > Buttons > there you can configure the Buttons style: https://ibb.co/2S0PdVT
Moreover, this Add to Cart button style is specifically linked to Product Styles which you can change by going to the Dashboard > Theme Settings > Products archive > Product styles > there you can change the Product Styles: https://ibb.co/R6GH07j
Best Regards