The search function is a pain in the butt.
Anyway, I just figured out how to style wp-signup.php for Arras, specifically, and thought I'd share with other network site users.
1. Download wp-signup.php from your server.
2. Create a backup copy of this file, in another folder. DO NOT EVER DELETE THIS FILE.
3. Open wp-signup.php in a text editor, preferably something like Notepad++.
4. Go to line 62, which reads:
<div id="content" class="widecolumn">
Replace it with:
<div id="content" class="section">
Press return to create a new line and insert:
<div class="entry-content clearfix">
5. Scroll all the way to the bottom of the file. At line 454 you'll see the following:
Copy one of those and create a third closing div tag directly below them, so that you see this:
6. The last line of the code calls the footer. Right above it, insert the get_sidebar code, so that it looks like this:
<?php get_sidebar(); ?>
<?php get_footer(); ?>
7. Save the file and upload it to the root directory of your WordPress installation, overwriting the previous version.
8. Go to http://yourdomain.com/wp-signup.php and view the changes. It now displays within Arras correctly. See an example here: http://www.danbirlew.com/wp-signup.php
If you ever switch to another theme besides Arras, you must revert to the old wp-signup.php. Upload the backup I told you to make in step 2 above.