Terms:
There is a very strict difference. A ‘template’ (in the themes ‘templates’ folder) is the code file that generates the forum display. This is akin to a wordpress theme template.
An ‘overlay’ (in the themes styles/overlays folder) is a CSS styling layer that sits on top of the themes main css (or in this case php/css) file.
We do need to keep these disparate terms separate and clear and use the right one at all times.
Plugins:
I am afraid I agree with you (sorry Steve). I think it does and will lead to a certain measure of confusion with the less savvy user. Not fond of ‘Add-ons’ myself I have to say. My own personal favourite would have been ‘modules’.
Codex/using Themes:
Apart from the confusion with the template/overlay term in the first sentence (oh and a spelling error!) then this looks like a grand start to me. Two things. Don’t mark this page as complete as it will need updating when we know the url of the download which, of course will not be the current beta location. And at the bottom I would just add that the overlay can be selected at any time not just when you activate a theme.
Other than that – looks good to me – even with those Windows images 
Need to bring Steve in on this for consensus but on the Codex themes menu I would re-order the left hand column to be ‘Using Themes’, ‘Theme Structure’, ‘Creating a Theme’. And personally I would remove ‘Core Themes’ as I am not sure what benefit that gives us.
What I think we SHOULD mention on the ‘Using Themes’ page is the other menu items. Needs a paragraph on setting a Mobile Theme and just a one liner on using the theme editor. And then I would think it’s done.
Overlay Images:
This brings up another point as well and I will open a ticket for this. The main theme images are probably out of date and need re-capturing and I think we should state in the theme description which overlay was used for the screenshot.
I would like to have small thumbnails of each overlay. A small portion of each (same portion in all cases) just to show the main colouring. But I think this really needs to be in the theme admin section not in the codex. Maybe on a popup or something. If you fancy creating the images I can do the coding. I would go for quite small ones though – say not bigger than 200px…. But if it doesn’t get done it is not the end of the world. Just a very nice to have.
What’s Next:
I can’t remember now if you said you still had the text for the Creating a Theme entry that you wrote before. If you have and there is any chance you could put that back in place that would be great. I was planning on doing the ‘Theme Structure’ myself as my next Codes foray but if you get there first they hey! I wont complain 