I haven't been using safari for long and this appears to be only in the newer version from what I can see online.
When using the inspector I cannot add chained rules (IDK the technical term for them…something like this…
#mydiv *
Which is selecting everything after the #mydiv. I have poked around the settings and I have been trying to search, but
Best Answer
One option is to first focus on an element that would satisfy the rule (this is because the rule list only shows rules that apply to the current element and the second you would input a rule that does not it would "disappear")
Then you simply go to Elements Tab -> Styles -> Rules and click on New rule. Input your new rule (you can edit both the selector and the rule).
Alternatively you can create a new rule (+ New Rule) which is automatically created on the current element. Fill the CSS and then change the selector.