Bootstrap theme: editing the registration page I am new to Drupal and wondering how to edit the look and feel of the registration page form? For what it's worth, I'm using the Bootstrap theme. I'd usually do this via CSS; however, with Drupal this is somewhat tricky. The problem is the form fields are expanding to fill their container, and, as you can imagine, it looks pretty ridiculous having a username field filling 3/4 of the screen. Looking at the elements class shows no clues as it is not a standard Bootstrap class (must be Drupal generated I imagine). I have edited the basic page content type in an attempt to make it work - but as I have learnt: the registration page is not styled from basic page. Any help would be great, thanks!
Take a look at drupal api documentation. Customizing forms is quite easy.
https://api.drupal.org/api/drupal/modules%21system%21system.api.php/function/hook_form_FORM_ID_alter/7

Just hook into that user_registration_form and for example add into class index bootstrap classes col col-lg-*. Or you can add #prefix and #suffix to wrap things into columns etc. And install Devel module to examine form's structure.

I'm not sure if it possible to use page--user--register.tpl.php to modify layout. Possibly you could create that and add col col-lg-offset-* col-lg-* to whatever element is the container for page[content] variable.