Change to Light,Dark theme in Windows 10 Regedit:
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Choose Light or Dark Theme Color for Default App Mode using  REG:

HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Themes\Personalize

AppsUseLightTheme DWORD

0 = Dark theme
1 = Light theme

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Choose Light or Dark Theme Color for Default Windows Mode using  REG:
This option is only available starting with Windows 10 1903.

HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Themes\Personalize

SystemUsesLightTheme DWORD

0 = Dark theme
1 = Light theme

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From:
https://www.tenforums.com/tutorials/24038-change-default-app-windows-mode-light-dark-theme-windows-10-a.html

Date Found: 11th September 2019