You can use CSS to change the design but it would be recommend to implement that in the custom.scss as a own class.

Hi

Yes, you can able to format the buttons.

In the styles section of the button just use border-radius CSS style.

For example: Add these css styles 

border-radius: 1500px;

height: 100px;

width: 100px;

The Output will be like this 

The height and width of the button depends on the caption of the button

If this needs to be done for every button in the project you can also just override the default mendix buttons in your custom CSS like this:

.btn { new css }

and then compile it with calypso.

This saves you the time for adding an additional styling class on every button.