Jon all you would need to do is make the template and css changes via the update package http://www.phpbb.com/downloads/olympus. ... 0.8&code=1
Every phpBB update, you would just update the template and css files via that update package.
I did take into account people that were using edits within their template files but it seemed phpBB were going with template inheritance more and changed the style to pretty much keep up.
In some ways adding the template changes yourself is easier if you are confident to make changes to files, because it removes the wait for me to update the template and for phpBB to validate the update. This way, when phpBB update, all you need to do is download the edits via their update link and apply the changes.
REMEMBER - for template updates, download the file with ftp to your computer, add the changes in notepad+ or similar and then upload the changed file again by ftp. Changes made in ACP to template will go when you purge the cache, so best to make changes locally on the file.