Docs
This article also answers:
Just log into your TEST site and navigate to Settings » Checkout and Self- Serve Portal (under Customer-facing essentials) » Themes (on the left panel). Now download the current theme, navigate to the Themes page on your LIVE site and upload it.
On uploading, the default version of your theme will be applied and the customizations wouldn't show up. You will have to manually apply the custom version by clicking on Configure & Publish Theme on the themes page.
Here's a video for your reference.
Note: In case of V3 checkout pages, the theme settings are not available as the whole checkout process happens in the form of a pop-up box on a single page. Refer here to know more.
Articles & FAQ