gulp: simplify themes tasks

gulpfile.js

@@ -203,12 +203,9 @@ if (fs.statSync('.git').isDirectory()) {
 	gulp.task('buildHTML', gulp.series(prepare, 'compileStory'));
 }
 
-const themeTasks = [];
-for (const entry of fs.readdirSync('themes')) {
-	if (fs.statSync(path.join('themes', entry)).isDirectory()) {
-		themeTasks.push(makeThemeCompilationTask(entry));
-	}
-}
+const themeTasks = fs.readdirSync('themes')
+	.filter(entry => fs.statSync(path.join('themes', entry)).isDirectory())
+	.map(entry => makeThemeCompilationTask(entry));
 
 exports.html = gulp.series('buildHTML', moveHTMLInPlace);
 exports.themes = gulp.parallel(themeTasks);