Theme example which extend liferayThemeTasks by adding a new task to apply PostCSS, autoprefixer and cssnano.
For this some dependencies have been added: https://github.com/marcoscv-work/styled-postcss-theme/blob/master/package.json#L25-L27
And PostCSS as devDependency: https://github.com/marcoscv-work/styled-postcss-theme/blob/master/package.json#L17
New task is declarated here: https://github.com/marcoscv-work/styled-postcss-theme/blob/master/gulpfile.js#L14-L22
Following original API documentation: https://github.com/liferay/liferay-theme-tasks#api