For Opera, put the lines I've quoted above in a text file with .css extension, then right click on any forum page -> Edit site preferences -> Display tab -> My stylesheet field, select the the created .css file here.
For Firefox, install this extension, open its options, press "Write New Style", fill in any name and put this in the large text field:
@-moz-document domain('forums.perpetuum-online.com') {
body { background: #111 none !important; }
.brd .item-subject .hn a, .brd .post-author a, .brd .post-byline a { color: #fff366 !important; }
h1, .blue { color: #fff366 !important; }
.post-byline, .post-link { color: #ddd; }
}
then "Save".
For other browsers, try googling for "per site user css".