Do you find background animation distracting? Here is how to disable them!
Guide to Disable Particles in Background
Find tetra online in your steam library, right click, properties.
Go to tab Local Files and click on Browse Local Files.
Go to TetraOnline_Data folder and find file called"sharedassets1.resource" and delete it.
Before and after
Why Does It Work?
Because this file is just a .webm video. So in theory you should be able to replace it with some other animation, but I haven't tried it.