Well, that's an incorrect reason, because obviously Reaper is using dark mode for the save/load project dialogs
If I disable dark mode, those dialogs go back to the drab old gray.
So, conclusion is that dark mode is not UWP-only, because default File Explorer in Windows is not UWP either. Which means Win32
might have some API for this (or, in some cases like with those file dialogs, there's an automatic conversion). Which means if that's true, Reaper can use that. I'm spitballing, of course, but this seems plausible.
UWP Explorer looks like
this.
EDIT: After a bit of googling, while the state of light/dark mode is not directly exposed to Win32 through an API, a Win32 program can query registry:
Code:
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Themes\Personalize
REG_DWORD AppsUseLightTheme = 0x00000000 // dark mode for programs
REG_DWORD AppsUseLightTheme = 0x00000001 // light mode for programs
REG_DWORD SystemUseLightTheme = 0x00000000 // dark mode for OS chrome
REG_DWORD SystemUseLightTheme = 0x00000001 // light mode for OS chrome