Highlight pinned topic?

I think, but I’m not sure, that the only items in the topic list that have the class .has-excerpt are the pinned topics. You could try adding this to the admin>customize>css/html area

.list-container .has-excerpt .title {
    display: inline-block; // makes it easier to position
    background: yellow; // gives the title a yellow background
}
// gives the pin icon a yellow background and pads it out to match the title
.has-excerpt .topic-status {
    display: inline-block;
    background: yellow;
    padding: 15px;
}

If you figure out how to use the developer tools in your web browser you can play around with it until it works.