[Dead Eddie]

Try adding this to your additional.css template:

.postcontent::selection{
background:#cc0000;
color:#fff;
}

.blogcontent::selection{
background:#cc0000;
color:#fff;
}

.postcontainer::selection{
background:#cc0000;
color:#fff;
}

.postcontent::-moz-selection{
background:#cc0000;
color:#fff;
}

.blogcontent::-moz-selection{
background:#cc0000;
color:#fff;
}

.postcontainer::-moz-selection{
background:#cc0000;
color:#fff;
}

Should work until vB decides to change those classes.