Where to place icons and other resources in SpringBoot app?

I am not a newbie in Vaadin, but recently have started developing with SpringBoot.
And now I don’t know where to put icons and other theme resources for usage in my project.
Also, I don’t know how to use custom css files.

Please, help me.

Create theme folder and files as described here
and store the theme folder in src/main/webapp/VAADIN/themes