gulpfile.js

var gulp = require('gulp'),
    sass = require('gulp-ruby-sass'),
    autoprefixer = require('gulp-autoprefixer'),
    minifycss = require('gulp-minify-css'),
    rename = require('gulp-rename'),
    replace = require('gulp-replace'),
    livereload = require('gulp-livereload'),
    lr = require('tiny-lr'),
    server = lr();

function replaceWithAssetURL( match, imgurl){
  return 'background:url({{ "'+ imgurl + '" | asset_url }})';
}   

gulp.task('styles', function() {
  return sass('assets/_scss/') 
      .on('error', function (err) {
          console.error('Error!', err.message);
      })
      .pipe(autoprefixer())
      .pipe(minifycss())
      .pipe(rename({extname: '.css.liquid'}))
      .pipe(replace(/background:url\((.*?)\)/g, replaceWithAssetURL))
      .pipe(gulp.dest('assets/'));
});

gulp.task('watch', function() {
  // Listen on port 35729
  server.listen(35729, function (err) {
      if (err) {
        return console.log(err)
      };
  
      // Watch .scss files
      gulp.watch(['assets/_scss/*.scss','assets/_scss/**/*.scss'], ['styles']);
    });

});