Restructure and add build process

.gitignore

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+# Ignore NPM dependencies
+node_modules/
+package-lock.json
+
+# Ignore build process output
+dist/

gulpfile.js

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+/**
+ * The Gulpfile is the file that tells the task runner, Gulp, what to do.
+ *
+ * The file is split up into a series of tasks. The task named 'default' will
+ * be executed when the command 'gulp' is run on the command line. The NPM
+ * script 'build' (defined in package.json) will run gulp to build the project.
+ *
+ * Gulp.js: https://gulpjs.com/
+ */
+
+const gulp = require("gulp")
+const del = require("del")
+const postcss = require("gulp-postcss")
+
+/**
+ * Removes any previous builds from the distribution directory.
+ */
+gulp.task('clean', async done => {
+  await del("./dist/")
+  done()
+})
+
+/**
+ * Copies legacy pre-built CSS files from older versions of the codebase
+ * directly to the distribution directory.
+ */
+gulp.task('legacy', () => {
+  return gulp.src("./legacy/*.css").pipe(gulp.dest("./dist/css/min/"))
+})
+
+/**
+ * Takes CSS files, minifies them, and outputs them to dist/.
+ *
+ * This task uses PostCSS, which is a CSS plugin that can be configured to do
+ * many different things. PostCSS is configured in postcss.config.js.
+ *
+ * PostCSS: https://postcss.org/
+ * PostCSS plugins: https://www.postcss.parts/
+ */
+gulp.task('css', () => {
+  return gulp
+    .src("./src/**/*.css")
+    .pipe(postcss())
+    .pipe(gulp.dest("./dist/"))
+})
+
+/**
+ * Copies other files from src/ to dist/, which makes them available for
+ * deployment.
+ */
+gulp.task('files', () => {
+  return gulp
+    .src(["./src/**/*", "!./src/**/*.css"])
+    .pipe(gulp.dest("./dist/"))
+})
+
+/**
+ * Runs all tasks in order.
+ */
+gulp.task('default', gulp.series('clean', 'files', 'legacy', 'css'))

package.json

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+{
+  "name": "common-theme",
+  "scripts": {
+    "build": "gulp"
+  },
+  "devDependencies": {
+    "del": "^6.0.0",
+    "gulp": "^4.0.2",
+    "gulp-postcss": "^9.0.0",
+    "postcss": "^8.2.4"
+  }
+}

postcss.config.js

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+/**
+ * PostCSS configuration
+ */
+module.exports = {
+  plugins: []
+}