Add dark mode configuration setting in UI tab

- Allows to choose "Auto", "Always On" or "Always Off"
  - Dark mode options are only shown if the style does not support theme
- If Auto is chosen, value is retrieved from OS
  - On Windows, the application needs a restart to apply the settings
- Use the default dark palette for Windows, like on Linux

src/common/settings.cpp

@@ -67,6 +67,7 @@ SWITCHABLE(u8, true);
 // Used in UISettings
 // TODO see if we can move this to uisettings.cpp
 SWITCHABLE(ConfirmStop, true);
+SWITCHABLE(DarkModeState, true);
 
 #undef SETTING
 #undef SWITCHABLE

src/common/settings.h

@@ -90,6 +90,7 @@ SWITCHABLE(u8, true);
 // Used in UISettings
 // TODO see if we can move this to uisettings.h
 SWITCHABLE(ConfirmStop, true);
+SWITCHABLE(DarkModeState, true);
 
 #undef SETTING
 #undef SWITCHABLE

src/common/settings_enums.h

@@ -155,6 +155,8 @@ ENUM(ConsoleMode, Handheld, Docked);
 
 ENUM(AppletMode, HLE, LLE);
 
+ENUM(DarkModeState, Off, On, Auto);
+
 template <typename Type>
 inline std::string CanonicalizeEnum(Type id) {
     const auto group = EnumMetadata<Type>::Canonicalizations();