How to change style of horizontal line

How can I change the style of the horizontal line form the “Insert” menu?

You can check this:

How can I do that in “Look & Feel” >> “Themes” … as default value for my wiki?

I’m afraid there’s no such option there, either you can edit your skin or create a new one.
For that, you can see: Skins (

Thanks. I will have a look.