file (required, string) is the path to the file within the system.
What kind of path, within what system? It's not clear that the 'file' type couldn't just be another kind of 'link'. If various fields like 'background' were defined to be URLs, that would offload a lot of complexity onto existing web specs.
> cover fills the entire width and height of the node.
Does that work like the CSS background-size: cover; or background-size: 100% 100%;?
> ratio maintains the aspect ratio of the background image.
Does that mean CSS cover? contain? Something else?
Colors can be specified in hex format, e.g. "#FFFFFF". Six preset colors exist, mapped to the following numbers:
1 red
2 orange
3 yellow
4 green
5 cyan
6 purple
Maybe the file format isn't meant to reproduce the exact same look in different software, but merely communicate user intent. Your guess is as good as mine.