A couple of things....first I have a centered image in a simple form in the header section. In the form builder it is centered, but on the web it is not. It is stuck to the left. This brought up a question for me....
How can I gain access to the HTML and/or CSS files to correct for any marginal condition that is causing this?

Secondly, concerning form validation...
Coffee Cup has some limited form validation, but I'd like to use some advanced validation using JQuery. This would require I access the HTML page. I can view the source, but I can't modify it. Please advise. Thanks!

We need to see the site in order to be able to do more than making guesses. Please post an URL

I've seen this problem on several of my forms. It's very random. I can sometimes get it to correct itself by removing the image and saving the form then close and reopen the app and replace the image.

Ok, Instead of posting the image as a background image in the header section, I used the logo element and it seemed to fix the centering issue. It stays in the center of the header now. However, there definitely seems to be a bug in form builder when using a single instance (no repeat) of a background image desiring it to be "centered" in the header section. This sticking "left" in the header happened on every single one of my forms I created.

The file is an .FB file (I assume is form builder). What would be the easiest way to modify this HTML? Straight away in a browser? Or can I import it in to the HTML editor? Is it even possible?

You should be able to alter the html to get it corrected. I would open a support topic so the CC gurus can take a look at your .fb file.