After your launch and your site is live, making changes can be risky if you aren't careful. A careless change in the Code Editor could break whatever page you're working on, whether it be your main index, your subscribe flow, or your checkout page. To make editing a live site safe, we've added the ability to copy, edit, and preview themes--this way you can edit your theme and only publish the changes to your live site when you're sure you have it the way you want.
When editing your site after launch, the safest thing is to use the copy feature. This will make an exact copy of your theme, including all images and styles. You can then edit the unpublished copy without it affecting your live site at all.
You can preview the unpublished them by clicking the thumbnail on the left or using the "View" button in the Designer.
Once you've made your changes and confirmed that it's the way you want, click Publish and that copy will become the live version of your site. Your customers will experience no down-time and won't see your half-complete changes.
You can also export a theme. This will generate a zip file containing all the html, css, js, image, and other files of your theme for you to download and modify locally.
Sometimes the visual Designer will create "dynamic patches" that aren't included in your theme source. These are not included in the theme export.
You can have as many themes installed as you like, and as many copies of a theme. However, if you have a copy you no longer need and don't like it cluttering up the theme list, you can delete any unpublished theme.
You cannot undelete a theme.