The color for fill passage feature can be set with transparency (%). when doing so, the borders of the areas don't get the transparency setting (and I can't find a way to correct this afterward). As a consequence, all the areas filled with the feature have a none-desired plain border.
This isn't a display issue as the border is exported with *.SVG files.
Something that could be improved on the same transparency matter is the final color rendering. If we apply some transparency, the area become transparent but the color gets actually ligther (on a white background). I wonder if there is a way to keep the expected color rendering with transparency. I suppose this is more complicated than said.
Filling areas are drawing with border lines to guarantee complete filling. With tranparency colours the intersection of line and filling area cause a colour change. Unfortunately this could not be prevented.