Changing themes made my original custom theme disappear

After trying the themes, I wanted go back to my custom one I’ve been using for years, and discovered that when I returned to “default” theme, it was literally the default white one, and not the one I’ve been using all these years (this one: … layout.htm).

Does this mean I was supposed to save my custom theme before changing it to one of the new presets? Do I now have to completely redo mine from scratch again?

If you didn’t have a save file of your appearance options, then yes I’m afraid you will have to reset them. Switching themes does return everything to default build for that theme, including the Default.