In the Pro version, the search form accessible via Appearance -> widgets is pre- styled which is the reason it is behaving like that. You can place it in the sidebar or footer widgets to see how it appears by default.
Search form in the menu bar is entirely possible by removing searchform style css completely but it is not recommended because we are not sure how it will behave in the responsive environment. If you have implemented it already let me know the URL so that I can have a look.