I work with a CSS expert who has never worked with vaadin.
He uses to work with a classic scss compiler which provide the source map option.
So he can easily debug its page in FF or chrome by binding SCSS and CSS file.
it’s possible to use an alternate SCSS/SASS compiler such as Compass.
How to integrate this into your workflow heavily depends on the build
system you are using (e.g. if you use Gradle, there’s a gradle-compass
plugin at https://github.com/robfletcher/gradle-compass ).
I have at least seen Valo-derived themes that were compiled with Compass. Of course things could have broken with later Vaadin versions. What kind of errors are you getting?
Oh, and do note that to use Compass you will need to extract the Vaadin theme files out of the JAR file since Compass does not know to look there like the Vaadin compiler does.