@d-healey Cool thanks, I found it in other post:
LAF.registerFunction("drawThumbnailRange", function(g, obj)
{
g.setColour(0xFFFFFFFF);
g.drawRoundedRectangle([a[0], a[1], a[2], a[3]], 5, 1);
});
It only modifies the color of the range thumbnail but for some reason it also makes the range background transparent :)