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: https://www.xwiki.org/xwiki/bin/view/Documentation/UserGuide/Features/XWikiSyntax/?syntax=2.1&section=HorizontalLine

Thanks.
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 (XWiki.org)

Thanks. I will have a look.