When setting up a multi-step form, the form fields all render at the same time and thus the steps are broken.
In order to render multi-step forms correctly, the injectFormOwnStylesheet must be set to true if a cusom stylesheet isn't declared. By setting this value to true, forms uses its default style sheet.
To make this change, edit the forms.config in the following location: ~\modules\_protected\EPiServer.Forms
If the changes don't appear right away, try recycling the app pool or perform an IIS reset to clear the cache.