The value
HKCU\Software\Microsoft\Windows\CurrentVersion\Themes\Personalize → AppsUseLightTheme (DWORD): 1 = apps default light, 0 = dark. Its sibling SystemUsesLightTheme drives the shell's own surfaces (taskbar, Start), so the two can diverge deliberately.
Who listens
- WinUI 3 / UWP / Win32 apps using the Modern.Xaml theming stack pick it up at the next theme-invalidated repaint.
- WinForms and older GDI apps keep whatever colors the developer painted — they were never subscribed.
- Browsers often re-read it on a fresh navigation, not on the flip — F5 answers “but nothing changed.”
The value is per-user, free of side effects, and instant-revertible — a model citizen of per-account tuning.
Where it fits
Dark/light is the tamest tweak in the catalog: a personal value whose restore is literally rewriting the remembered default. It's part of why the visual preset's undo is exact rather than approximate.
ProofTune ProjectEngineering notes — every claim here names the bytes a real tool touches. Verify first, install second.