Add an enum to identify themes

We can add here all the built-in themes, plus a custom one that could be loaded from a file.

common/themes/theme_id.h

@@ -0,0 +1,10 @@
+#ifndef THEME_ID_H
+#define THEME_ID_H
+
+enum class ThemeId {
+    Classic,
+    Light,
+    Dark,
+};
+
+#endif // THEME_ID_H