Fading in two directions with unshaded middle - Faded rectangles create "lines at boundaries"

I’d like to do some fading of my background page from the middle to top and bottom – so far as good. My Problem is that I don’t want to start the fading immediatly (e.g. the upper third is faded, the middle third is background color, the lower third is faded).

So my suggestions are two ways:
a) Manipulate somehow the fading speed (I read the corresponding thread but was not able to do the necessary adjustments) and work with "color top", "color bottom", and "color middle".
b) Draw two rectangles at the top and the bottom which are faded in one direction.

My problem: While a) looks "good" but has no extended middle background color part, b) creates some mysterious lines in my output file at the boundaries (the lines are not printed in texmaker but in all of my pdf viewers):

Below a minimal working example for version b:

documentclass{report}
usepackage{tikz}
usetikzlibrary{calc,fadings}
definecolor{colorBackground}{RGB}{65,110,131}

begin{document}

begin{tikzpicture}[remember picture,overlay]
  fill[color=colorBackground] (current page.north west) rectangle (current page.south east);
  fill[top color=colorBackground!0, bottom color=colorBackground] (current page.north west) rectangle ($(current page.north west)+(paperwidth,-10cm)$);
  fill[top color=colorBackground, bottom color=colorBackground!0] (current page.south west) rectangle ($(current page.south west)+(paperwidth,+10cm)$);
end{tikzpicture}

end{document}

Kind regards!

documentclass{report}
usepackage{tikz}
definecolor{colorBackground}{RGB}{65,110,131}

begin{document}

pgfdeclareverticalshading[colorBackground]{pageshade}{100bp}{%
  color(0bp)=(white);
  color(25bp)=(white);
  color(40bp)=(colorBackground);
  color(60bp)=(colorBackground);
  color(75bp)=(white);
  color(100bp)=(white)
}

begin{tikzpicture}[remember picture, overlay]
  shade [shading=pageshade] (current page.north east) rectangle (current page.south west);
end{tikzpicture}


end{document}