I feel like this should be straightforward but by all accounts seems to cause a lot of problems and is steeped in black magic..
I have developed a custom theme, I want to use this theme on several websites, and I want to be able to update the theme occasionally and have these updates pushed automatically or manually to those websites.
I’ve explored a variety of similar questions/answers here, as well as various plugins and tutorials to this effect, all unfortunately to absolutely no avail.
What do I need to include in the theme code, and what is the correct method of pushing an available update?
EDIT: Just to clarify, I don’t want this theme to be public or hosted on the WP repository.