docs: public static void registerTheme(String name, Theme theme) {
          if(theme == null) {
              throw new IllegalArgumentException("Name cannot be null");
          }
          else if(name.isEmpty()) {
              throw new IllegalArgumentException("Name cannot be empty");
          }
          Theme result = REGISTERED_THEMES.putIfAbsent(name, theme);
          if(result != null && result != theme) {
              throw new IllegalArgumentException("There is already a theme registered with the name '" + name + "'");
          }
      }