why is windows changing theme while installing an update

Hello, thank you for reaching out to Microsoft Q&A.

May I please confirm if you’re experiencing the same issue when installing all Windows updates or only specific ones?

Reverting to the default Windows theme can occur after major updates, such as future updates, are installed on the device. However, if this happens after every update, it could be due to a corrupted user profile or a sync issue.

That is, if you have another Windows device signed in with the same Microsoft account and it’s using the default Windows theme, the settings might sync automatically each time you sign in after updates are installed.

Alternatively, the user profile on this device may be corrupted and causing the theme to reset repeatedly.