uisettings: Add enumeration type for themes

Eliminates the usage of a magic number to indicate the default index of the themes array,

src/yuzu/uisettings.h

@@ -29,6 +29,15 @@ struct Shortcut {
     ContextualShortcut shortcut;
 };
 
+enum class Theme {
+    Default,
+    DefaultColorful,
+    Dark,
+    DarkColorful,
+    MidnightBlue,
+    MidnightBlueColorful,
+};
+
 using Themes = std::array<std::pair<const char*, const char*>, 6>;
 extern const Themes themes;