Change/Edit Theme

Hi,
after a long time without Vaadin today i try to create a new Web-App with Vaadin.

But i run in a problem.
I use IDE IntellJ Idea (version 15.0.2) and Vaadin 7.
I try to edit some Theme styles but i cant find a Web-Content or VAADIN folder with the Themes. 

This is my Project in Idea:

How can i create a scss File to edit the style of my application?

To change existing Themes I use

@Theme("valo")
public class MyVaadinApplication extends UI {
...
}

Thanks for help.