Not a silly question at all! Themes and theme components are very similar overall, and you can even convert a theme to a theme component with this convert button on the theme’s admin page.
So instead of forking, you can convert Welcome Page to a theme component and add it to Minima. That way you can still track Welcome Page updates.
@angus I’ve made a PR to change this to a component by default
https://github.com/angusmcleod/discourse-welcome-page/pull/1