z-index for different clusters

[raph-tr]

hi
i show on my map 3 different pruneClusters and cannot find a way to set the z index offset for each cluster : i wish one of the three should always show on top and can't achieve this either by myCluster.setZIndex() or by playing on the css, which seems overwritten as far as zindex is concerned...
any clue ?
the page : pgearth.dev