Hi Jess, you're very welcome. I guess everyone may do things differently or quite commonly face inexperience barriers. I do understand your confusion and I'll try to clarify...
I use FTP direct access to my online files, I have no patience logging in and fishing around via the file manager in the host's control panel. Even at that, I'd probably guess that different hosts may have different set ups. It might be the same...
I log into my host account and choose 'File Manager' from the 'Site Management' menu. I then click on my domain directory folder, then I click on my Zen folder(test shop is in sub folder). I click on the listed folder named 'includes', 'templates', and then my currently named template folder. Then I'm left with my main template folders. Then, you can open up the corresponding folder to access your stylesheets(css folder). You have the option to download a relevant file to your desktop and edit the file with a text editor or edit the file with the host's editor, without downloading/uploading to the desktop. I believe it best to download/upload, then you can make a copy of the original in case of foul ups with customizing steps or at least download an original copy of the relevant file for back up, if you choose to use the host's editor. Anyways, for your case... there should be three stylesheets in your 'css' folder with the stylesheet names stated prior.
I'm not sure how much you paid for your store layout and it's none of my business, but three stylesheets is not very good to have. I could understand two, as per the original Classic template... even at that, your custom template should have been designed based upon the default template. So, I have to relay that having three stylesheets is bad practice. Your current design does not warrant more than one stylesheet.css . :no:
I must make aware that the current layout is busted, due to the center picture being too big for the constraints of the overall template width. I also recently upgraded to v1.3.6 and logged in today, without a notice to upgrade further. Are you sure your version isn't v1.3.5 or lower maybe?