Use Default Colorful theme by default outside of Windows

On OSes with system-wide theming this allows yuzu to follow system style, regardless of its exact coloration, working well with both light and dark system themes. Dark /Colorful, on the other hand, forces dark theme regardless of user preferences set in system settings, making for a poor default.

Use Colorful variation to keep in line with icon style of patron-voted Dark Colorful.

src/yuzu/configuration/config.h

@@ -48,6 +48,14 @@ public:
     static const std::array<int, Settings::NativeKeyboard::NumKeyboardMods> default_keyboard_mods;
     static const std::array<UISettings::Shortcut, 21> default_hotkeys;
 
+    static constexpr UISettings::Theme default_theme{
+#ifdef _WIN32
+        UISettings::Theme::DarkColorful
+#else
+        UISettings::Theme::DefaultColorful
+#endif
+    };
+
 private:
     void Initialize(const std::string& config_name);