Your configuration is just a little off. minify
is also just a target name. Do this instead:
cssmin: {
minify: {
src: 'path-to/default.css',
dest: 'path-to/default.min.css'
}
}
OR
cssmin: {
my_target: {
src: 'path-to/default.css',
dest: 'path-to/default.min.css'
}
}
Please read http://gruntjs.com/configuring-tasks on how to configure tasks.