You can easily add animated background to any block that allows changing the background. Here's how you can do it:
1. Navigate to the block where you want to change the background.
2. Hover over any space to trigger the Block settings menu appearance.
3. Click on the icon Edit image and choose the animated .gif file on your computer. Please note that the file size should not exceed 5 Mb.
4. Your image will be automatically resized and added to the background of your block. Some templates will automatically add a color overlay to match the look and feel of your site.
You can read more about adding an animated background in this tutorial.