Greetings, as you all know the default settings (registry) of a user profile is stored at "C:\Users\Default\NTUSER.DAT", everytime a new user log in, Windows will duplicate this registry setting and assign it to the new user right? Now for example, I want Windows to use dark theme by default for every users, of course including the one I create during oobe phase. So in host OS, I go to regedit and select HKEY_USERS, load hive, choose the "\Users\Default\NTUSER.DAT" inside the install.wim of my desire mounted ISO, create a name "TEMP" for the registry key, go to "HKEY_USERS\TEMP\SOFTWARE\Microsoft\Windows\CurrentVersion\Themes\Personalize", change the value of DWORD "AppsUseLightTheme" to 0, select "TEMP" again and unload hive. After that I tested it in VM, but neither the profile I created during oobe phase, nor the user profile I created after oobe phase, use dark theme right after log in, and the "AppsUseLightTheme" is still 1 when I view that in HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Themes\Personalize. I wonder why the settings don't apply to new profiles created?
Newer W10 releases and W11 block many Default User registry entries during new user provisioning. There's nothing you can do, except to apply those HKCU changes AFTER the first logon. Unfortunately, there is no guide to which reg keys are blocked. But it's happening for more and more user environment settings. The way to test is update Default\NTUSER.DAT as you've been doing, and create a new local account to check what's inherited.