Answering my own question:

Apparently, you can't do that in any easy way. The Demo pages are intended ONLY as a sandbox for trying the system's features, and you can't get access to the actual page files (the CSS page or the .html files for the individual pages) in order to copy them to a new host.

After wasting much effort, I have a couple of suggestions for anyone in a similar position.


(1) DON'T DO THAT

Do not invest a great deal of effort creating a site on the Concrete5 Demo system. You'll waste most or all of your time. The purpose of the Demo system is merely to help you decide whether Concrete5 is the system you want to use to develop your permanent Web site. Go ahead and play with it to see if it's a system you'd like to keep using; once you've made that decision, stop using the Demo.

If you do think Concrete5 is the way you want to go in building a real Web site, then you will need to find a place to host that site permanently, and get Concrete5 running there. These are the options detailed under the "Download" link at the top of the Concrete5 site pages. (Basically: you can pay the Concrete5 organization to host your site and provide tech support in developing it with the Concrete5 software, or for about 1/10th the cost you can get a regular Web ISP service to host the site and run the Concrete5 software but not help you use it. You can still come to these forums to ask questions either way.)

Once you have decided that you do want to continue using Concrete5, and have made a decision which hosting option to use, go ahead and get set up on that host, and begin developing your permanent site using Concrete5 at your permanent host.


(2) IF YOU DID THAT

Oops.

You already got sucked into the Demo system and built a beautiful site under Concrete5 that almost nobody can find and is going to disappear into thin air in a week or two. You goofed. Like me. What can you do?

You can't simply move the site to a new host. You can't get access to the template files needed to do that. What you have to do is . . . start over. Yeah, it's painful, but there's no other way.

You have to get a permanent hosting setup, as mentioned in (1) above, and as explained on the "Download" link at the top of the page. (NB: If you're having this problem, you're clearly not a candidate for "Option B" - running Concrete5 on your own Web server. So you're looking at paying someone to host your site, running Concrete5 on their server: either the Concrete5 organization or a private ISP. The latter is cheaper, but you don't get tech support for Concrete5. Luckily, you do still get to use these forums.) Decide how you want to do it, set up a hosting contract, and make sure (if you're not hosting with Concrete5 itself) that your host has installed Concrete5 in your new account. Also make sure you know how to log into your account, and how to access Concrete5 in that account.

Now you need to recreate your work in Concrete5 at that new permanent installation. Since you can't just transfer your Demo files over there, what you want to do is maximize the return on your Demo effort, to salvage as much as possible. Here's what to do:

(a) Go to your hosted Concrete5 account, select the template you want to use (presumably the one you were using in your Demo pages), and set up the site there the same way you did here.

(b) Go into Dashboard on your hosted account and make all the settings you need (colors, titles, etc.), the same way you did on the Demo (if you did - otherwise, just set it whatever way you like).

(c) Using "Add Page" on your hosted account, create new pages on that site, one for each page on your Demo site. Set the templates for each page to be the same as the template for that page on the Demo.

(d) Go to each page on your hosted site, one at a time, and use the editing features to add/remove page blocks, headers, sidebars, etc., to replicate the design of those pages as you set them up on the Demo.

[You will notice that so far you haven't used any of the work on your Demo site. You're just re-doing all the same effort on the hosted site. That's right. This is the part - the template setups - that can't be copied, because you can't access those files on the Demo. You have to just do it all over from scratch. But now that you have the site structure set up, you can copy some of the content.]

(e) Go into "Dashboard/File Manager" on the Demo site and "Download" every file or image you have stored there. (Click the check-box at the very top of the file listing; each and every check-box below it should then be checked. Select "Download" from the drop-down menu, and download the files onto your hard drive. Note that it only downloads the files ON THAT DISPLAY PAGE. If you have more than one page of files in "File Manager", you have to do this again on each page to be sure you have downloaded all your files.)

(f) Go into "Dashboard/File Manager" on your hosted site and UPLOAD all those files, so you now have them available in "File Manager" on your permanent Concrete5 installation.

(g) For each page on the site, access that page in "Edit Mode" on BOTH the hosted site and the Demo site. (I.e., open two browser windows, one for your Demo Web site and one for your new hosted Web site. Display the same Web site pages on each one.). For each template content block on the page (header, sidebar, text block, image block - each of the red-bordered content blocks on the page), go into "Edit" in the Demo page and select/copy all the content in that block. Go into "Edit" in the same block on the hosted site page and paste that content into that block.

Do the same for every block on every page. This will move your content without having to retype everything. It should also preserve most of the formatting of the content. When you "Update" and "Publish My Edits", the pages on your hosted site should look almost exactly like the same pages on the Demo site. Keep doing this for every page, and you will eventually have reproduced the Demo site on the hosted site.

IMPORTANT TIPS:

When uploading images or other stored files, or linking between pages on the site, use the "Add Image", "Add File", and "Insert Link to Page" functions at the top of the edit screen - DO NOT JUST PASTE IMAGES OR LINKS FROM THE DEMO SITE. If you do copy/paste content that includes images or linked text, you must DELETE ANY COPIED IMAGES and UNLINK ANY LINKED TEXT WORDS, then re-insert those images and links using the link functions on the edit screen. (The reason is that the links to images, files or pages contained on the Demo pages are identified in HTML by reference to their location at the Demo page site. Those files and images will be unreachable from outside the Demo site, and the page links will be broken when your temporary Demo site goes down. This is why you had to copy all the stored files and images over to your new site in Steps (e) and (f) above - so you could re-link them at the new site.)

If the new pages do not look exactly as they were on the Demo site, you can open the template content blocks on the page in "Edit" mode and use the "HTML" function to reveal the page coding. If you know HTML, you may be able to edit this to create the formatting you want. Otherwise, just use the regular editing commands on the new hosted site pages to adjust the look and formatting.


FINAL:

Yes, this is a huge pain, and it requires re-doing a lot of stuff you've already done. (See #1 above.) It will go faster than the first time, because now you know the system, and you're just re-creating what you have done before, not trying it out and making new decisions at every step of the way. But it's a big waste of time, no doubt, and the larger and more complete your Demo site was, the more time you waste. (See #1 above.) Sorry about that, but at least you do get to salvage some of what you've done.

Good luck!