How can I sync/update custom theme across my Grav sites?

So I am running 5+ Grav sites (yes, that's how much I love Grav!) and I use the same custom theme I developed. How can I update the theme across all sites without manually replacing files every time over and over again (it's a time-consuming and prone to an error so this is a really poor solution)?

The only thing is - I use specific homepage template that is different for each site. But blog/article post template and page templates are pretty much same.

Git or rsync for installations on different servers. Symlinks for instances on the same machine.