Say, something like this:
Which would be given with
article.group-bb-writers div.topic-body {
background-color: rgb(202, 233, 207);
}
The existing per-user customization, the highlighted name and title, might not be enough for some people: Reddit - Please wait for verification
(that subreddit is a little bit, ah, crazy with its CSS customizations…)
div.usertext-body div.md *:first-child::first-letter {
text-transform: lowercase;
text-decoration: underline;
}