- You can add another Theme (probably a clone of your ‘normal’ one) - call it Test or something.
- Keep your normal one as the ‘default’.
- Make sure you include the exact same Theme Components
- Select this one for your admin user
You can then muck about with this one as much as you like. Others can (in theory) select the Theme but they are very unlikely to unless signposted to it clearly.
Just remember to select the ‘normal’ one when you are done!