setting transparent background in tile mill so that base map is the background and when toggling layers

Question

Does anyone know how to make the background of a map transparent totally ,transparent in tile mill.I tried removing the background color rule in the carto css map section but that did not work,instead it just leaves a plain white background.I tried playing around with polygon opacity attribute between 0.1 and one but I still see a white background. I need it to be totally transparent so that I only see the markers and I want my background to be my base map.

Please see image attached , that white is what I am trying to get rid of.

Answer 1

go to project settings and change format to png , it works.

Answer 2

It appears the problem was to do with multiple symbols within the carto css . The second symbol style always overrides the the first