I have a development store that has two themes (the current published theme and the old theme). I wanted to preview my old theme- so I pressed preview under the options (I didn’t publish or replaced the new theme). Then, a few hours later I looked at my store and saw that the old theme is the theme that is been shown even though I didn’t publish it. Why would it show the old theme? So even if I don’t publish the old theme but press preview it will automatically replace the new theme?
just trying to understand how it works...
I just tried to preview a theme on my development store and I noticed the same thing as you. If you open your store in incognito mode you will see the old theme, so you can rest assured that nothing has changed to the end customers.
All you have to do when you're done previewing a theme is clicking the "Close Preview" button at the bottom of the page: