Smooth UI is not better than other libraries like Material UI or Ant UI but the focus is different. Smooth UI put the focus on developer experience, extending a style is very simple. Also it exposes some classes, you don't have to learn any API to extend components, you can simply inspect components and use the classes!
Looks like the example uses that because it would otherwise be overridden by the variant=primary (which seems to be the default) so explicitly overriding that variant would probably be more intuitive and not require the !important hack.
An example of extend: https://codesandbox.io/s/7k8o4x7lj6