I was able to find the SASS scss files for the layout, but not for the theme.Full SaSS Support
Scss syntax of Sass is used to create the css of
both layout and theme for easy customization
theme.css seem to be provided from the primefaces jar (primefaces-6.1/META-INF/resources/primefaces-omega/theme.css). This becomes clearer when reading the documentation (I did so for installation purposes)
I would need the theme SASS files for the omega theme.Theme
Omega layout depends on the Omega theme of PrimeFaces that is available since 6.0. Setup is simple as defining primefaces.THEME context parameter in web.xml such as "omega".