If you are on any Arch Linux derivative distribution (eg Manjaro, BlackArch, Archcraft) you can choose to install the 'Opera' package from the official Arch Linux repositories using the commands 'sudo pacman -Syu opera' and 'sudo pacman -Syu opera-ffmpeg-codecs'. Of course you can also install it directly from your default package manager (in my case pamac.manager)

###########################
; This file is stored in UTF-8 encoding

[Info]
Name = Start of Xfinity
Author = G. Testing
Author URL = https://github.io
Version = 2

[Start Page]
Background = Xfnfinity_intro.webm
Position = center bottom
Title Text Color = #FFFFFF
Title Text Shadow = #757575
First Frame Image = Screenshot.jpg

[Web UI Pages]
Background = Screenshot.jpg
Position = center bottom
###########################

Thanks to this is the base file that I found in some Opera forums, I was able to run my custom theme without problems within Opera GX, but unfortunately I couldn't modify the background of the "GX Corner" tab.
Analyzing the HTML code I could see that it takes a screenshot of the video.webm file but I would like to use a different image for this browser tab.
<video disablepictureinpicture="" loop="" src="blob:https://gx-corner.opera.com/f51e828a-ae26-44ba-b018-4fd584da9746" class="svelte-xtsafi" style="">< /video>

@pinportal It worked pretty well on ArchLinux. It didn't have the direct x86_64-linux-gnu so run these commands.
sudo mv libffmpeg.so /usr/lib/libffmpeg_h264.so
sudo mv /usr/lib/opera/libffmpeg.so /usr/lib/opera/libffmpeg.so.orig
sudo ln -s /usr/lib/libffmpeg_h264.so /usr/lib/opera/libffmpeg.so
Thank you very much