Every theme is made up of a .zip file, Opera classifies this as a Wallpaper Archive. The .zip is made up of 4 files: 2 are images, 1 is a video, and 1 is a text file.

Starting with the first 2 image files, we'll call one "FF" and the other "BG". It is not important if the images are the same or different. What is important is that the FF image is the same as the First Frame of the video you would like to use as a theme. BG is only used as a thumbnail for the Opera addons store. What you name these files does not matter as long as they are not the same file name.

Next is the video you would like to use as a theme, it must be in the .webm format. What you name it does not matter, and the resolution should be at whatever scale you would like to support.

Lastly is the text file, this must be a .ini file, and it must be called persona.ini. This is the file that will either make or break your theme. Here you will describe to Opera how your previous 3 files will be recognized and positioned, as well as any incidental information.

Now, Step by Step on how I make a theme:

1. Find a video you would like to use as your theme
   Optional: make sure it loops perfectly for seamless playback
2. Screenshot the First Frame of the video in question, and copy it with a different name, keep both copies
   Optional: take a second screenshot if you want a different thumbnail
3. Create your .ini file, open notepad and use this format just as you see it. Anything in quotes replace with your own info or it will not work! Your version should not have the quotes.
   For name put the title of the theme
   Author put what you would like to known as
   Author url is optional unless you want to share a personal website or the like
   As you can see I have labeled my files differently, but like I said the name does not matter.

[Info]
name = "4K Tree Bird"
author = "K-DOG9000"
author url =
version = 2

[Start Page]
background = "video".webm
position = center bottom
first frame image = "2".jpg

[Web UI Pages]
background = "1".jpg
position = center bottom

4. After your done putting in your information click File > Save as > Save as Type > All Files (.) and name it persona.ini
5. Highlight your 4 files > Right Click > Send to > Compressed (Zipped) Folder. Name it whatever you want.
6. Upload to Opera, your .zip files should now work for yourself or for anyone else if up loaded to the store.

Things to keep in mind:
Opera will not upload anything bigger than 50Mb
I suggest using .jpg as the image format to save on space, and keeping it to a max resolution of 1080p even if the video is 4K
If you need help converting an image format I use Photoshop but Gimp is a free alternative
if you'd like your video to loop I recommend Premiere but you can probably get by in Windows Movie Maker or Video Editor which should be included on any Windows Machine
If you need to convert the video to .webm I use handbrake a free tool that will convert pretty much any video to .webm, I suggest keeping the bitrate around 6144Kbps and using 2 passes, even for 4k footage to save on space
Remember the more space it takes up the worse your theme will run on slower machines.

That's all I can really think of now, I hope I helped even a little bit, I know most of this information isn't easy to find, but good luck and have fun!