Hi Nathan,
You can add your class(button-class) with custom styles inside the main.css file, it will work as your expectation.
The best way to add your custom styling is to add the new class inside the custom.scss file which will be available in the following folder inside your project theme/styles/web/sass/app/custom.scss and compile your project file using css compiler
Here is one css compiler documentation : https://docs.mendix.com/howto/front-end/calypso
Thanks!