How do I set an image as a page background?

To set an image (instead of a color) as a page background, add a T674 "Modifier: Adding a background image to the entire page" block from the "Other" category to the page.

Then the image you have added in the block will be assigned as a background of the page.

If you set a background color for some blocks on the page, it will overlay the background image. You can remove the background color from the blocks in the Block Settings: Remove the value from the color assignment field and save the changes. The block background will become transparent.