Hello I had copied the Proton Light Mix template Manipulated and saved with different names. You selected as the default template. Unfortunately, the new template is not in that "bx_menu_popup ('sys_switch_template', window);" not selectable.
Creating your own Template Module is probable the best way to maintain your theme changes. It will have its own folder structure, therefore una updates won't touch it. You only have to worry about updating version compatibility during the una updates, or if structural changes were made.