I checked where the wiki class is set when a first post is in wiki mode, but it’s not high enough in the hierarchy to work well.
I think the best way to do this is to create a tag named wiki (or anything else) and use the body class .tag-wiki before each css customization
Then you can work with the class
.tag-wiki .timeline-date-wrapper {
background: ****;
}
.tag-wiki .timeline-date-wrapper {
}