Move love.system.getTheme() to love.window.getSystemTheme().

Despite its name, `SDL_GetSystemTheme()` needs the video subsystem to properly function. So, put it in love.window instead.

src/modules/window/Window.cpp

@@ -112,5 +112,14 @@ STRINGMAP_CLASS_BEGIN(Window, Window::DisplayOrientation, Window::ORIENTATION_MA
 }
 STRINGMAP_CLASS_END(Window, Window::DisplayOrientation, Window::ORIENTATION_MAX_ENUM, orientation)
 
+STRINGMAP_CLASS_BEGIN(Window, Window::SystemTheme, Window::THEME_MAX_ENUM, systemThemes)
+{
+	{"unknown", Window::THEME_UNKNOWN},
+	{"light", Window::THEME_LIGHT},
+	{"dark", Window::THEME_DARK}
+};
+STRINGMAP_CLASS_END(Window, Window::SystemTheme, Window::THEME_MAX_ENUM, systemThemes)
+
+
 } // window
 } // love

src/modules/window/Window.h

@@ -114,6 +114,14 @@ public:
 		ORIENTATION_MAX_ENUM
 	};
 
+	enum SystemTheme
+	{
+		THEME_UNKNOWN,
+		THEME_LIGHT,
+		THEME_DARK,
+		THEME_MAX_ENUM
+	};
+
 	struct WindowSize
 	{
 		int width;
@@ -252,11 +260,19 @@ public:
 
 	virtual void requestAttention(bool continuous) = 0;
 
+	/**
+	* Get information about the system theme.
+	*
+	* @return System theme, which can be either light, dark, or unknown.
+	**/
+	virtual SystemTheme getSystemTheme() const = 0;
+
 	STRINGMAP_CLASS_DECLARE(Setting);
 	STRINGMAP_CLASS_DECLARE(FullscreenType);
 	STRINGMAP_CLASS_DECLARE(MessageBoxType);
 	STRINGMAP_CLASS_DECLARE(FileDialogType);
 	STRINGMAP_CLASS_DECLARE(DisplayOrientation);
+	STRINGMAP_CLASS_DECLARE(SystemTheme);
 
 protected:
 

src/modules/window/sdl/Window.cpp

@@ -1655,6 +1655,25 @@ void Window::requestAttention(bool continuous)
 	// TODO: Linux?
 }
 
+Window::SystemTheme Window::getSystemTheme() const
+{
+	SDL_SystemTheme sdlstate = SDL_GetSystemTheme();
+	SystemTheme state = THEME_UNKNOWN;
+	systemThemes.find(sdlstate, state);
+
+	return state;
+}
+
+EnumMap<Window::SystemTheme, SDL_SystemTheme, Window::THEME_MAX_ENUM>::Entry Window::systemThemeEntries[] =
+{
+	{Window::THEME_UNKNOWN, SDL_SYSTEM_THEME_UNKNOWN},
+	{Window::THEME_LIGHT, SDL_SYSTEM_THEME_LIGHT},
+	{Window::THEME_DARK, SDL_SYSTEM_THEME_DARK},
+};
+
+EnumMap<Window::SystemTheme, SDL_SystemTheme, Window::THEME_MAX_ENUM> Window::systemThemes(Window::systemThemeEntries, sizeof(Window::systemThemeEntries));
+
+
 } // sdl
 } // window
 } // love

src/modules/window/sdl/Window.h

@@ -24,6 +24,7 @@
 // LOVE
 #include "window/Window.h"
 #include "common/config.h"
+#include "common/EnumMap.h"
 #include "graphics/Graphics.h"
 
 // SDL
@@ -133,6 +134,8 @@ public:
 
 	void requestAttention(bool continuous) override;
 
+	SystemTheme getSystemTheme() const override;
+
 	void handleSDLEvent(const SDL_Event &event);
 
 private:
@@ -191,6 +194,9 @@ private:
 
 	Uint32 dialogEventId;
 
+	static EnumMap<SystemTheme, SDL_SystemTheme, THEME_MAX_ENUM>::Entry systemThemeEntries[];
+	static EnumMap<SystemTheme, SDL_SystemTheme, THEME_MAX_ENUM> systemThemes;
+
 }; // Window
 
 } // sdl

src/modules/window/wrap_Window.cpp

@@ -765,6 +765,15 @@ int w_getPointer(lua_State *L)
 	return 1;
 }
 
+int w_getSystemTheme(lua_State *L)
+{
+	Window::SystemTheme theme = instance()->getSystemTheme();
+	const char *str = "unknown";
+	Window::getConstant(theme, str);
+	lua_pushstring(L, str);
+	return 1;
+}
+
 static const luaL_Reg functions[] =
 {
 	{ "getDisplayCount", w_getDisplayCount },
@@ -809,6 +818,7 @@ static const luaL_Reg functions[] =
 	{ "showFileDialog", w_showFileDialog },
 	{ "requestAttention", w_requestAttention },
 	{ "getPointer", w_getPointer },
+	{ "getSystemTheme", w_getSystemTheme },
 	{ 0, 0 }
 };