Extend Theme to Different Content
Worked this one out after a bit of fiddling, so for others trying to figure it out too...


First find the template file you need to override - if it's part of a module, it might be in the module templates folder. Otherwise install Theme Developer - it will help you track down the template that's being used for that content https://www.drupal.org/project/devel_themer
Chances are you'll need to override that template with one from your theme - identify the template theme that you want to use (easiest done using Theme Developer on the page that is coming up how you want), then copy that template file from your theme templates folder ([theme].tpl.php) to your sub-theme templates folder
Rename the template file in your sub-theme templates folder the same as the one you want to override (as identified in step 1), and voila.
(optional) you might find a CSS file somewhere is still screwing up the way your content looks (in my case it was in the module css folder) - you can either tweak it with other tools like CSS Injector, or copy the offending CSS file into your sub-theme css folder and tweak it there.
If you are overriding CSS, you'll need to identify the CSS file in the sub-theme .info file (found in your sub-theme folder) - add the following line: stylesheets[all][] = css/filename.css  (where filename.css is the name of the tweaked CSS file) under the line stylesheets[all][] = css/custom.css