Something like:
.myButton{
background-color: orange;
}
.myButton:hover{
background-color: green;
}
Do it in the .scss files and compile it using gulp,koala, calypso,… Never change styles in the .css files directly. They will be overwritten when someone changes the scss files.
You can apply your own css classes to all the objects. Double click on your button, there should be a “class” input field.
In the custom.scss file, you can now add your custom css code.
Use calypso for custom css and it is very easy to use, follow the link instruction to use calypso (https://docs.mendix.com/howto/front-end/calypso). Give classname to the button in mendix e.g. myButton and write css code in theme\styles\web\sass\app\_custom.scss file.