Many of the styling properties of Mendix widgets can be changed by setting new values on variables in custom-variables.js in the theme.
Some variables may not yet exist, open the styling of Atlas_Core and look them up in variables.js.
You can overrule other styling by exporting a class with the same name from your theme or a UI resource module that has higher priority than Atlas Core. When you right-click your theme module you can mark it as UI module. By default, it will get a higher priority than Atlas_Core