I had the same issue. Sometimes it works with quotation marks (e.g. "image_width": "700",), sometimes not. I also got it to work without quotation marks and simply removing the space between the colon and the number: "image_width":700, See if that wo...

Hey not sure if you solved this, but the file you put the code in is incorrect. There should be a file called theme.css or similar...the code should go in there 