change bullet points to hollw gray circles

slides.tex

@@ -2,6 +2,9 @@
 
 \usepackage{alltt}
 
+\usepackage{color}
+\usepackage{graphicx}
+
 \hypersetup{
   pdftitle={Compiling code and using MPI},
   pdfauthor={http://scitas.epfl.ch},
@@ -13,6 +16,9 @@
 \author{\url{scitas.epfl.ch}}
 \date{} 
 
+\defbeamertemplate{itemize item}{hollow circle}{$\circ$}
+\setbeamertemplate{itemize item}[hollow circle]
+\setbeamercolor{itemize item}{fg=gray}
 
 
 \begin{document}