Change user experience player in flow for a project that is already in development

You can switch a Project to a different Theme at any time. There's an option on the Home tab called Change Theme, as well as on the Design tab.

So if the Theme already exists, it's as easy as using that feature to make the switch. Here's a lesson on how to Set your project to use a different Theme (Change Theme). 

If the Theme doesn't yet exist (say you want to match your own branding instead of the colors in the default Theme options), then Administrators can make new custom Themes while Authors can make Variations of an existing theme. A Variation is a color change-only, it can't alter any of the functionality of the theme itself. Here's more on the difference between Themes and Variants.

Hope this helps, let us know if you need more information!