You can use the css property 'border-radius’ for that: https://www.w3schools.com/cssref/css3_pr_border-radius.asp
Yes, you can use border-radius to 20px or something like that, but isn’t there a class in Atlas-UI styling for that? Hm, not here: https://docs.mendix.com/howto/front-end/styles#5-buttons. Hm, Bootstrap to the rescue:
This makes the button a circle, just like you asked, but not conform to your sample buttons. So, no, border-radius it is.