It probably just wasn't featured. That page doesn't include every Safari extension ever published.

Thanks for your reply Mr. Weinstein! I watched your WWDC talk like 20 times while developing my app 😅. Here is the feature request ID: FB8853796. To expand further: My app extension, Keys, injects elements and would like users to be able to customize the color of these elements within the app (if they prefer the colors pink and purple say). Moreover, I would like to add a dark background behind the elements I inject so that they are easier to see for those with vision impairments. Several people in my App Store reviews have complained about these elements being difficult to see and about the necessity for a background to improve contrast, but I wouldn't want to do this for all users and would only do it if it were an option people could elect to toggle. If you want to see a video demonstration of Keys to see what I mean, there's one available on my YouTube channel "patrickshox".