Hi,
I suspect it would not work as audit mode is not used for preparing account profile configuration, as Microsoft official article states, it is used for Install applications, add device drivers, and run scripts.
We could configure ThemeName, DefaultThemesOff in Unattend.xml which would be useful to configure default.
Please refer to: How to create an unattended installation of Windows 10 for more information about unattended file.
Also, if your devices have been deployed, we could add a change default theme script to GPO when clients boot up.
Please refer to: Using Startup, Shutdown, Logon, and Logoff Scripts in Group Policy
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Bests,