How to define the width of signup & register pages

Hello,

Using New Blog Templates an Pro Sites I would like to center and define the width of the wp-signup and registration pages. I was previously using this css:

#content {
width: 980px;
margin-left: auto;
margin-right: auto;
}

But found that it effected the ability of the rest of my site to be responsive. I then used this css:

.mu_register {
margin: 0 auto;
width: 860px;
}

It works on the main wp-signup.php pages, but there are still some other pages like wp-activate.php that are still full screen.

I'd like to find a solution that makes sure the width is defined for all pages.

Here is a staging site url where you can see how the forms and content are full width:

http://myfitbodybiz.staging.wpengine.com/wp-signup.php

Thanks, Chris