placing Vaadin theme jar within vanilla Vaadin SPA

I created a vanilla Vaadin SPA with the It Works button. Following the example for Vaadin 8, which ran OK until I copied the three beans and modified the UI.java to match, now the following error occurs.

INFO: Requested resource
[/VAADIN/themes/mytheme/styles.css] not found from filesystem or through class loader. Add widgetset and/or theme JAR to your classpath or add files to WebContent/VAADIN folder.

Any suggestions greatly appreciated.