nah in style properties you can't but you can add a rule like this in extra.css ...
to style message boxes only in 'styling and customization questions' you can view the source of the page you are interested in and see what nodes its contained within (look at the <body> tag, the class names there are what you want to be looking at)
.node47 .message {
background-color: #C0C0C0;
}
if i wanted to make that apply to node48 as well for some reason i could do this...
.node47 .message, .node48 .message {
background-color: #C0C0C0;
}
There is a couple ways to go about it but the straight css mode is the fastest and easiest