Button's styling for Foundry

Hi there. I would like to modify styling for buttons in different Foundry stacks such as
Scroll To, Sort, Mega Modal and Flat Button 2 stacks.

I guess I would need to inject some code in order to modify the following :

  • Rounded corners.
  • Border colour (normal and on hover).

Do you have any idea and whereabout I should inject the code ? Thank you for helping !