[Tex/LaTex] How to draw only the border for the nodes in TikZ Mindmap

mindmapstikz-pgf

I was working on a PGF/TikZ Mindmap:

tikz mindmap

I cannot find a way to just draw the border, which looks like an easy action but I've tried everything! I manage to change the color of the nodes, but no control on the border. The code is this one (the same from the page above):

% Author: Till Tantau
% Source: The PGF/TikZ manual
\documentclass{article}

\usepackage{tikz}
\usetikzlibrary{mindmap,trees}
\begin{document}
\pagestyle{empty}
\begin{tikzpicture}
  \path[mindmap,concept color=black,text=white]
    node[concept] {Computer Science}
    [clockwise from=0]
    child[concept color=green!50!black] {
      node[concept] {practical}
      [clockwise from=90]
      child { node[concept] {algorithms} }
      child { node[concept] {data structures} }
      child { node[concept] {pro\-gramming languages} }
      child { node[concept] {software engineer\-ing} }
    }  
    child[concept color=blue] {
      node[concept] {applied}
      [clockwise from=-30]
      child { node[concept] {databases} }
      child { node[concept] {WWW} }
    }
    child[concept color=red] { node[concept] {technical} }
    child[concept color=orange] { node[concept] {theoretical} };
\end{tikzpicture}\end{document}

Best Answer

I'm not sure if this is what you want:

\documentclass{article}

\usepackage{tikz}
\usetikzlibrary{mindmap,trees}
\tikzset{concept/.append style={fill={none}}}
\begin{document}
\pagestyle{empty}
\begin{tikzpicture}
  \path[mindmap,concept color=black,text=black]
    node[concept] {Computer Science}
    [clockwise from=0]
    child[concept color=green!50!black] {
      node[concept] {practical}
      [clockwise from=90]
      child { node[concept] {algorithms} }
      child { node[concept] {data structures} }
      child { node[concept] {pro\-gramming languages} }
      child { node[concept] {software engineer\-ing} }
    }  
    child[concept color=blue] {
      node[concept] {applied}
      [clockwise from=-30]
      child { node[concept] {databases} }
      child { node[concept] {WWW} }
    }
    child[concept color=red] { node[concept] {technical} }
    child[concept color=orange] { node[concept] {theoretical} };
\end{tikzpicture}
\end{document}

output of code