UserAgent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.96 Safari/537.36

Steps to reproduce the problem:
I'm trying to make background scale to the browser window. I can set it manually on a new tab, but I haven't figured out how to store background size in the theme manifest.json file.
Attempted to add:
1. As sub-property of ntp_background_alignment as "right bottom/cover" and "right bottom /cover";
2. As sub-property of ntp_background_repeat as "no-repeat/cover" and "no-repeat /cover";
3. As separate "ntp_background_size": "cover" value in the properties section.

What is the expected behavior?
At least in 3rd case 'cover' property should be applied to background to make it scale to the window size.

What went wrong?
In cases 1 and 2 "/cover" part were ignored and adjacent value were reset to default one "right bottom/cover" to "right center" and "no-repeat/cover" to "no-repeat". Case 3 were completely ignored since chrome doesn't support such property.

WebStore page: https://chrome.google.com/webstore/detail/ditzy-doo-mod/hchkkjlghnjeniomalikelpgdpjocldk

Did this work before? No 

Chrome version: 58.0.3029.96  Channel: stable
OS Version: 10.0
Flash Version: 

I think it would be useful for many themes out there.