Minchau Dang describes the issue with the current 6.1 releases (i.e., until SP1 is released) ...
Right now, the generate:archetype for themes is incorrect.
After investigating, the reason is that the stuff that's in
src/main/webapps doesn't get copied over until the 'package' phase, while the build-css goal is run in the generate-sources phase. This means that everything in .sass-cache is correct EXCEPT for any customizations, making it largely useless for creating themes.
One workaround is to explicitly copy the files in src/main/webapps in any phase before 'package' and delay the build-css until a phase afterwards. One strategy I tested was to copy the files in the 'generate-sources' phase and to run build-css in the 'prepare-sources' phase.
The issue and fix commit (at least for 6.1.x) are here:
We need to describe what people using the current release archetypes should do to pick up this fix. Perhaps it is just modifying their existing POMs.
Developer references: Minchau and Mika, and ticket https://issues.liferay.com/browse/MAVEN-48