Please
check out this tutorial
I’ve written, which teaches you how to create a standalone Vaadin theme that can be reused in other Vaadin projects as an add-on.
Try it out and let me know what you think! 
a month back i read the article and was able to create theme jars based on my very customised css.
Thanks Marlon Richert for this neatly written very helpful article.
You’re welcome! Glad to be of service.