code: @NotNull
      @ObjectiveCName("editGroupThemeCommandWithGid:withTheme:")
      public Command<Void> editGroupTheme(final int gid, final String theme) {
          return callback -> modules.getGroupsModule().editTheme(gid, theme)
                  .then(v -> callback.onResult(v))
                  .failure(e -> callback.onError(e));
      }
docs: Edit group's theme
  
  @param gid   group's id
  @param theme new group theme
  @return Command for execution