On the ScreenFony.com site there's a quick tutorial showing you how to get started with Symfony2+Twitter Bootstrap+Assetic+LESS in a basic application. Assetic is a library that helps with asset management and use and LESS is a CSS pre-processor that makes it simpler to work with your site's CSS.
Bootstrap is a well known and powerful front-end framework for fast prototyping, it uses LESS and it can be easily integrate in your Symfony applications with the help of assetic. In this post I’ll show how to: Install bootstrap in you Symfony application, load it using assetic, and compile bootstrap LESS files with lessphp.
Using Composer, creating a new Symfony2 project is just a single command away. The just update the "composer.json" and run the install to get the other needed libraries (LESS and the Twitter Bootstrap). They help you set up some Assetic filters for LESS and provide a simple page to output the Bootstrap in your header.