From the Help Center, Do M.b updates to theme files extend to untouched files in custom themes?:

Q: I am wondering if people who set and edit a custom theme from a particular existing theme get updates to the files they haven’t edited when, say, Manton makes changes to the theme itself, or if you’re basically “stuck” with whatever versions of those files existed when they created the custom theme?

A: Yes, updates to the original theme will come through.