Thanks for testing it out.

The problem with roads without border lines is that some of them (yellow tertiairy) will not be visible at all against this yellow background. If I make the background white, the unclassified roads are not visible anymore. Ok, I can change the colours of either one of them but then it doesn’t look like the Mapnik osm map anymore. So I have to seek a compromise, like a borderline only on the yellow roads.