Developing a new theme without deactivating my existing

Hi.

I’m currently using the Motif theme for my site but I’d like to switch to a new theme and be able to work on this while still running my existing one. How do I “save and activate” a new theme and work on it in the background/offline until I’m happy with it? And then how do I publish this in place of the existing theme?

Any help and advice is greatly appreciated.

Many thanks.

Kelly.

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