gulp-webserver livereload doesn't work with css-Files

Question

I'm using gulp-webserver.

If I'm manually editing .js or .html files in my build/-folder then livereload becomes active and refreshes the page. In opposite .css files are ignored - But why?

var webserver = require('gulp-webserver');

gulp.task('gulpWebserver', function () {
    return gulp.src('./build')
        .pipe(
            webserver({
                host: '0.0.0.0',
                https: true,
                livereload: true,
                open: 'https://localhost:8000'
            }));
});