Seaside has a convenient message WAComponent>>confirm: that creates a dialog with yes/no buttons, however there is no CSS styling on this component. Is it possible to add styling to this component or do I need to subclass WAYesOrNoDialog in my app to do that?
There are two ways to apply your styles to the WAYesOrNoDialog:
If your calling it from your top level component, then subclassing it is the way, but you'd still need to implement confirm:onAnswer: on your own component to reference you're own class.
If you call it from within another component (that is not top level), you can render the component that's going to perform the confirm:onAnswer: within a <div> in the parent component.