after some search, I understand now how does a style and a
vector object work together. It seems that a f.ex. rounded
rectangle is by default a groupe and so most of the styles in the
style panel are not working to fill the grouped shape.
So you must ungroupe this shape and the style can now apply
to the shape
BUT: If you applied a style to a shap und you have to resize
the canvas, the styl will be destroied :(
Just draw a rounded rectangle, ungroupe it, apply a plastc
style (like any glossy style). Than press resize canvas in the
options, you will see that the style is now gone.