Yeoman out of the box needs some tweaking in my experience. It's Grunt engine really wants to minify things - that's one of it's major value-adds. I'm pretty new to it as well, but I imagine the empty cssmin area in the grunt file is what's messing you up. Try putting something like this into your cssmin area of gruntfile.js -
cssmin: { dist: { files: {'<%= yeoman.dist %>/styles/main.css': ['.tmp/styles/{,*/}*.css','<%= yeoman.app %>/styles/{,*/}*.css' ] } } },That should do you give your css minification some teeth. And, then you would uncomment cssmin in build task at the bottom of gruntfile.js so it runs when you do 'grunt build' on the CLI.