In the Advanced tab of each element settings, you can add and remove CSS styles that will be applied to the part of the form described by the button text. There is also a global CSS styles section at Settings → Style → Global styling in the form builder. Styles added in the global section will be applied to each element in the form.
For more control, you can create a file named custom.css and place it into the /css/ folder. This file will be loaded after all other Quform related stylesheets on your website so it’s the perfect place to add in CSS overrides. If you use exactly the same selector as one discovered from a stylesheet loaded above it, for example a theme, the styles you add to this file will override the theme styles. Firebug is a great tool for quickly finding the selectors that are styling something. You can also style the individual elements in this file, by using the selectors found in the Advanced tab of the element settings. For more flexibility than this you will need to create a theme.