Your theme is based on a older version of Vaadin, e.g. because you only changed the JAR. Recompile your widgetset.
It might be harmful as styles might be missing or selectors might be wrong due to changes on the client-side. Those would lead to a “funny looking” or even to a “unusable” application.
By migration to vaadin7 you have to change complete VAADIN folder, it seems that your are still using old vaadin theme, or previous version stylesheet is just cached by browser.