Create a file named userstyle.css in the conf directory. In that directory place the following css:
pre { #D70751; }
You could make this more readable by using white for your text color (and bold for your font-weight):
pre.file {
background-color: #D70751;
color: white;
font-weight: bold;
}
The only problem with this is that all of your code blocks will have the same styling, even if you used geshi. You could get around this by using the <file></file> markup for your personal comments and using this in userstyule.css:
pre.file {
}
Then you could use <code></code> for creating geshi blocks and any other blocks which you don't want to have the red background.