Your unpublished themes can be found by clicking "Change / Add Themes"
To edit an unpublished theme, simply select the unpublished theme and start working!
When editing your unpublished theme's pages, you will notice the Visual Design Editor will only have a "Save" and "Save Draft" button. This is because you can not publish pages until a theme is published.
When an unpublished page is published, all Saved versions of the theme's pages will become published.
Updated almost 3 years ago