Created Themes come up blank.

Hey Nicky_S,

It appears that you're having a problem creating a template in the Keynote app on your Mac and iPad Pro. When you create it, it's only a blank slide, and we're glad to provide assistance.

Create and use templates and themes in Pages, Numbers, and Keynote - Apple Support -- Review the information in this article to see if you've missed anything. Be certain to confirm that everything is set up correctly by checking the steps under "Get started".

If nothing helped, see if restarting both devices resolves the issue:

Restart your iPad - Apple Support

Shut down or restart your Mac - Apple Support

Get Support -- If you're still unable to save a template, please contact Apple Support for further help. 

All the best.

Thanks for outlining the steps you've taken Nicky_S,

Keep in mind that you won't be able to change the theme of a presentation that's being shared.

That said, try the following steps, and let us know whether you get the expected result:

1. Choose File > Change Theme (from the File menu at the top of your screen), then select a new theme.
2. Deselect the “Keep your style changes” checkbox if you want the new theme styles to override your style changes.
3. Click Choose.

The following page offers additional information that may be helpful: Change a Keynote theme on Mac

Take care!