It's not uncommon to have multiple projects for the same brand or client. But after customizing the theme for one project, it is not available to select in a different project. Every custom theme should be added to the theme library so they don't have to be recreated again. Yes, I know, I can save a page from the custom-theme project, download it as an image, and then upload it to a new project in the theme wizard, but...wouldn't it be easier to just select it from the theme library?