How to setup a new WordPress theme without deactivating the current theme?

In most cases it’s easiest to use a staging site. You make a copy of the live site, activate the new theme, and customize it on the copy (the staging site). Once everything is working like you want, you then copy the staging site over onto the live site. You’ll just need to make sure that no one is making changes to the live site while you’re working on staging.

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