Just extract the zip file to to the "packages" directory in the root of the site, then go to add functionality in the dashboard and you should see it listed there to install.

Forgive me for sounding stupid, but where would I find this?

I wont be unzipping the templates until Sunday on another computer, and this will be my first time doing this.

Thanks.

well if you have a properly "packaged" theme, you can still place them in siteroot/themes/ and they will install.

so you unzip a theme, you'll see a tree like

/folder
-description.txt
-thumbnail.png
-main.css
-typography.css
-default.php
-view.php

It doesn't explicitly need to be in a package, but that is probably legacy at this point and might get deprecated, well not might, I am sure it will be eventually :)

So yeah for now unzip to siteroot themes and assuming the theme is valid it will still show up in the dashboard to install.

Ok thanks. Still confusing, but hopefully I'll figure it out.

well some non-packaged up themes are available on c5mix.com...at least last i checked they were.

Another way to think about it is to look at concrete/themes/default/*

the default file there is the same exact template one should use to make a theme from, so say if you renamed default folder to hamburger and uploaded it to siteroot/themes/ you would see a new theme to install, admittedly without changing description.txt it would say default and whatever it says on the next line underneath that, but that is a good starting point for understanding in concrete5 themes.

Sorry I don't know how to go vaguer :(

here is a quick way to tell if it is a package or not, if it has a controller.php it is a package, if it does not have one then it is just a theme so you stick it in the /themes folder