A quick guide to change the style of your wiki posts:
Change the background color:
In your /admin/customize/themes
add this CSS under Common > CSS
.wiki .topic-body .cooked {
background-color: #dcffdc; /*light green*
}
Change text size
If you want you can also change the font used or the size of the text, even the color
.wiki .topic-body .cooked p {
font-size: 18px;
color: green;
}
Add text
You can add a text to make clear to users that what they are watching is a wiki post
.post-info.edits .wiki::before {
content: "EDIT THIS WIKI POST";
color: green;
background-color: #d2e2d2;
margin-right: 3px;
font-weight: bold;
border: 1px solid green;
padding: 3px;
}