First of all, all 10 demos here are excellent and I believe they all should have a place in Opera GX. However, since this is a competition where only one can win the Audience's vote, I'll have to make a choice and here's why I'll be choosing that particular demo.

The point of a background track here is to get out of your way and be a part of your workflow while you browse the web. The track also has to be sleek and modern to match with Opera GX's modern looks. It shouldn't be too epic, or distracting. It should simply provide you with a feeling of your browser responding to you through the music.
All that considered, I think tracks like A, B, C, D and F should not be chosen because, they really are great pieces of music but they lack the lowkey-ness that they required for this competition. We're voting for the best song that can fit into GX's background music, not THE best music here. There's also the fact that B is very much banking on the success of Stranger Things theme over here.
E and J are very close to what I was looking for as they have that edgy and modern feel to their sound. I was very close to choosing either of them.
I was going pretty well until more elements were being introduced in it, thus taking away the simplicity of the song.
G was almost my final choice, until I realized that Lo-Fi hiphop beats don't suite Opera GX's style. They almost clash too hard. We have a modern browser on one and and a jazzy lofi beat on the other.
Ultimately, the best choice here should be H in my opinion. It doesn't distract you, the progression keeps you hooked to the music but also doesn't take away your concentration, it even has a very loopable structure.
And thus, I have chosen H to be my final decision