How do you change the default Matrix CSS classes? I am currently trying to change a class called 'form-button' that is putting an ugly white border around the input button used for our search function…
Any advice greatly appreciated.
Frontend CSS is controlled by your designs and not Matrix. Matrix only controls some styles in Simple Edit and all styles in the admin interface.
Check your design for any CSS files being loaded into it and change the CSS there. You can also configure the search button in any way you want in the design parse file, including making it an image. See the designs manual for information about parse file settings.