[Permission Required: Account Admin, Content Admin]
A theme is like an outfit you put into the site, and though it doesn't change the structure and layout, it highly impacts the branding and aesthetics of the site.
New sites automatically use the default theme set for the account, and so there's a high possibility that a lot of sites in your account are using the default theme!
When you edit a theme from the Themes dashboard, you'll see the published sites using that theme on the right pane, like the one shown below.
All of the sites on the right site of the theme editor, will definitely be affected if that theme got edited!
If you need to change the theme and apply the change/s to specific site/s, it is best to clone the theme first, apply it the intended site/s, and then start editing it.
To clone the theme:
1. Find the theme from the Themes dashboard.
2. Hover over the theme you want to clone/duplicate.
3. Click Duplicate.
4. Give the duplicate theme a unique name and you can also add an optional description.
5. Click Create button.
Note: You need to reload your site editor, to see your new clone theme from the theme selector.