I downloaded demo38 and want to create the minified files for css and js. Also installed npm, yarn and gulp-cli described in the documentation.
Changed it in gulp.config.js but it does not create any minified files, just the regular "uncompressed" css/js files.
gulp --demo38 --prod also gives no errors. If I turn debug on, I just see a bunch von rewrite-css messages (besides the normal messages). Nothing more.