Layout of Zen Cart

Is the layout of zen cart controlled completely from the CSS or is some of the layout determined within the php? Problem is I don't know php and the css dosen't seem to have very much styling in there. I can't find any positioning attributes is this handled when you set up the site in the admin area? I've only just started to look at zen cart so please forgive me if it's a really obvious question

Layout is controlled by:-

a) Database settings, controlled in the Admin Panel
b) Some php (eg: tpl_product_info_display.php - you can move the elements around)
c) CSS - the stylesheets
d) in a (very) few cases, embedded html and css (such as in e-mail messages)
e) bits of javascript (the pop-ups, for example).

So let's say that I want to change the size of the boxes on the login page. Right now there are two boxes - one for create an account and one to login. What I'd like to do is make the create an account box run across the full column (in the middle of the page) and then have a line break and make the login box be below the option to create an account.

To be honest, my true goal is that I am getting ready to install COWOA module and I'd like to have the COWOA option be more prominently displayed than anything else on the page. I'd like for the COWOA "guest checkout" to be at the top of the page and then below that I'd like to show the Create an Account and Login options.

I'll be able to figure it out if I can just figure out how to make the boxes take up the entire width of the middle column of my website. Just FYI, my website is at https://www.shoptomlinsons.com. Thanks in advance!

-ajcannon