.category-support .topic-post {
background: red;
}
does:
The category-specific class is found in the <body> tag.
If you want to highlight only the first post:
.category-support .topic-post:first-child {
background: red;
}
If you want to change the background of category-specific topics in a topic list, then the topic you linked is the way to do it.
.category-support.topic-list-item {
background: red;
}
does:
