How to update sub-themes?

Problem/Motivation

I am not familiar with subtheme operations.

I have installed 5.5.16 and am running it with a subtheme.
The main theme is uninstalled.

What steps are required to update to 5.5.17 using Composer?
Do I need to re-install the main theme once?
How will the sub-theme behave after that?

Thank you.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes