This seems to happen if you leave a property completely empty. At least a backround property. Haven't really tested it or seen it on anything else so far.
I got it on the header.background property when I changed the color to clear and had no image url or nothing else selected.
Did the same on content.background-color recently and got the same result.
To get rid of the message I just selected repeat without any image url. Just to have something there but still get a transparant background.
Since this only happens when debug mode is on I guess it isn't a huge problem, but still somewhat annoying.