perhaps this thread need an update , well , after 2 years. [RC 0.3.1]
to change the email message background color/colour when selecting/clicking on it from the default red to any color you want :
open and edit
skins/default/mail.css in this example I change from red(default #CC3333) to navy blue (the hex
#3A4F77 ) .
#messagelist tr.selected td
{
color: #FFFFFF;
background-color: #3A4F77; /**default #CC3333 **/
}