Okay, thank you, I found it. All I needed was the email-header.php reference that you offered. For some unknown reason, I hadn't looked into the other php files in that same folder: email-addresses.php, email-footer.php, etc. You're right, some of the styles are called there.
BUT, WHAT A MESS - They're approach to formatting the email is half-baked at best.
They define a H1, but nothing else, especially H2 and H3 tags which are used throughout the email template. I had to go back to email-header.php and define a h2 and h3 code set, then hit every email template file that called them out with their special php code. For instance, anywhere you want a styled H2, you need to use the code:
<h2 style="<?php echo $header_content_h2; ?>">
And back in email-header.php, you need to define h2 with code that looks like this:
$header_content_h2 = "
color: " . esc_attr( $base_text ) . ";
padding: 12px 10px;
text-shadow: 0 1px 0 $base_lighter_20;
Which again, is copied from the H1 which is the only style they defined.
Basically, their email template was way too "big" and open (spaced out) - even the simplest order took up 2 pages to print and the client wanted to get it down to one (I know that every email client and print out is going to be different) but I had to agree generally speaking.
All in all, I had to tweak 5 files to get some simple style changes.
In the days when much of these (wordpress) sites are styled and formatted via the Admin Panels, or at least in CSS, it's frustrating to see hard coded, inline CSS buried within random files that you must then take a parent/child approach to formatting.
Thanks again for your help.