Aha, I obviously wasn't paying attention - I thought that was the version of my theme.

OK, so opera will load the updated theme now, but when I try to upload it I get:
Invalid persona.ini file, position property is invalid in section Web UI Pages.

I removed the offending line, tested, and tried to upload, and it gave me:
Invalid persona.ini file, position property is invalid in section Start Page.

I removed that line too, tested, and tried to upload, and it worked.

Using that format gives me a theme that doesn't work - I try to drop the zip file into Opera's themes dialog and it silently refuses to do anything: no error message, no output in the console, and no theme.

Here's the content of the updated persona.ini, conforming to the link you posted:

[Info]
Name            = Tool - Smokebox
Author          = AntiSol
Version         = 1
Author URL		= http://antisol.org

[Start Page]
Background		= smokebox.webm
Position		= center

[Web UI Pages]
Background		= smokebox.webm
Position		= center

I tried again, got the same message

Hi,

I just tried to upload an animated theme and got the error:

Theme background image smokebox.webm is corrupted.

The theme works just fine for me in Opera 32 on Linux. I'm guessing perhaps the website hasn't been updated to allow uploading animated themes yet?

the theme is available at http://antisol.org/opera_smokebox_theme.zip