are you looking to stile the button differently when is disabled or do you want to prevent form submission if a required field is missing?
If you use a button type submit inside a form container and all the input are in the form container as well and the workflow is triggered on submit (instead of on click) the form will be submitted only when all the required fields are filled. You get this automatically. Also see this post.
You can also add a disabled attribute on the button and bind the value to a formula that returns true if all the input values are not empty. You can use the same formula to bind the style of the button to add a visual clue to the disabled button.