Question about updating theme files

Hello, I'm coming from the WordPress world and so just have some questions to figure out...

I was thinking to build starter theme package that initially would be composed of 2 different layouts. Over time, I want to add additionally layouts and allow my existing client to be able to update his theme files to include the newer files.

Is it possible that when the user uses the "update" button in the admin section, that this can update the theme files as well? If not, could you explain how I could get someone to update the theme files?

Thank you

Grav has built in support to inherit a base theme (unlike WordPress) so you can do create a base theme that your custom themes inherit: