listings: set background color to gray:20

my-listings.sty

@@ -174,7 +174,7 @@
   commentstyle=\itshape, showstringspaces=false}
 
 \newcommand{\lstusecolors}{
-\lstset{backgroundcolor=\color{gray!10}}
+\lstset{backgroundcolor=\color{isae@bggray}}
 
 \definecolor{lstred}{rgb}{0.6,0,0}
 \definecolor{lstgreen}{rgb}{0.25,0.5,0.35}