how do we Customize the backend with Bootstrap :) ?

Can you be a little more clear. Bootstrap is only a front end framework backend frameworks would be something like rails and there are other great ones depending on the language you are using which is much more different than using bootstrap. You may be able to use angular if you are trying to build a single page web app of somekind. What exactly do you want to do?

If you are talking about Wordpress only you can use php. Many times there are ready made widgets that can make your life simpler and you barely have to do much work to get them running.

If you mean the Admin area, I think it would be time consuming to do this since the admin area comes with classes and Ids by default. If you are trying to put new columns to the admin area check out the following plugin:

wordpress admin style plugin

If you want to learn how to use it check out the this training video

If you want to change the look and feel, then check out Customizing the Wordpress Admin Area

Oh ok, I just saw your new post, what you could do is tell the admin area to load a new style by creating the following functiong in your function.php

function my_admin_style(){

wp_enqueue_style ('bootstrap' ,  get_template_directory_uri() . ' /css/bootstrap.css');  

}

add_action ( 'admin_head' , 'my_admin_style' );

Then go where the admin area files are and replace default classes with bootstrap.