There are a lot of different approaches to this issue, and you’ll need to find the method that works best for you (I list 3 below). That being said, offloading changes to Code Injection or just keeping things in separate files when possible will always make your life easier.
For keeping up with changes, all Ghost official themes are open source, so you can see the changes on their GitHub pages. For Headline, you can see that the Readme file was the most recent update. Clicking on the ## commits will show the history of theme changes.
Methods for keeping up to date
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Manual
Watch GitHub to see the changes and then manually copy those changes into your own theme. This is the most straightforward and often the simplest way of going about things. Themes usually change incrementally, so this method isn’t as tedious as one might expect. -
Use software to detect changes between your theme and the official one
@mjw mentions using an app called Meld for this. More details here: Make changes to a theme to upgrade easier? - #2 by mjw -
Fork the official theme in GitHub
This is the most advanced technique and requires some fluency with GitHub. This site goes over the basic process: How to build on top of the Casper theme in Ghost
If anyone has any other ideas, please shout 