For more information and installing Node.js please see its page at nodejs.org
It's optional and you don't need to use it.
I've used it for compiling LESS files into CSS and Mustache templates into static HTML

Less is a CSS pre-processor which is useful for creating and maintating large amounts of CSS code
You can find more info about it here lesscss.org
Almost all Ace CSS files are generated from compiling LESS files
First you need to install less compiler via npm: npm install -g less
To compile LESS files, you can run the following commands:
lessc ace.less path/to/ace.css
lessc skins/skins.less path/to/ace-skins.css
For options such as compressing output, etc, you can use its help by using the following command:
lessc --help
You can also use the in-browser CSS builder tool to build a custom CSS.
You can also use grunt tasks to build CSS. grunt less:main

Ace's demo pages are mustache templates compiled into HTML
For more info about Mustache files please see Mustache files
PHP Mustache is used during development.
And Javascript Mustache compiler is used for final demo output
For more info please see the following links:
Mustache
Mustache.js
Hogan.js
PHP Mustache

Grunt tasks are used to automate certains tasks such as compiling LESS files, etc...
For more info please see gruntjs.com

Tools