Using either Safari 11.0+ on Mac OS X 10.12.6 or Safari on iOS 11.0+ (earlier versions are not affected)
Hi, username! > My Dashboard > Skins > Create Site Skin
Scroll down to the "Advanced" section
Next to the word "Advanced," there should be a button-style link labeled "Show ↓" that makes it possible to access the advanced options.
There is no "Show ↓" link, so you can't access the advanced options.
Firefox 55 and Chrome 60 and 61 on Mac OS X both have the "Show ↓" option.
If you’re on iOS, Chrome and other browsers (e.g. DuckDuckGo) will also be missing the option.