How do I add a custom-built theme to Keynote for iCloud? I'm just after finishing the creation of a custom theme for my wife's new company. We created it with Keynote, and then backed up the .kth extension file and sent it from her computer to mine.

She's able to load it from her Keynote app on her OS X El Capitan. I was able to open it via email, and add it to "My Themes" onto my macOS Mojave, and later opened it on the iPhone and added it to "My Themes".

Then I decided to go and check out http://keynote.icloud.com .. And can only see Standard and Wide - but no My Themes.

So, how would I go about adding a custom-built theme so that it is available via Keynote for iCloud?

Is this even supported by the web-app Keynote for iCloud? 

I tried dragging the .kth file into the Keynote for iCloud file browser, nothing happened. Then I tried clicking "Upload" and selecting the .kth file, which resulted in the following error:



Cannot upload this type of file to this App Library.
Only files that can be opened by keynote can be uploaded here.


Which is somewhat bemusing, since Keynote can open the file. Just not Keynote for iCloud.

Any ideas?
From research across many forums, I cannot find any indication that Keynote in iCloud, as of 2019-11-01, supports Keynote themes.   

Hopefully, in the future, there will be a more positive answer to the question