Color gradient animated background glitch

Hello everyone,

I want to modify the background of my NodeRed dashboard with the node in order to make the interface more user-friendly.

I can modify the background without problem by creating a fixed gradient image like below :

But when I want to integrate an animated gradient background like below, it doesn't work, do you have an explanation on the code anomaly or the location in which I put it:

Do you have a tip to give me?