I installed as admin, although it does this anyway. Seems to be sticking at the moment. Thanks for the comment.
It's embedded in different sizes into C:\Windows\SystemResources\Windows.UI.Logon\Windows.UI.Logon.pri. There may be some tool that can dump PRI (package resource index) files. I used a file carving tool to extract the images.
Code: Windows Registry Editor Version 5.00 [HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Themes\Personalize] "AppsUseLightTheme"=dword:00000000