child theme of 2013 (arrows ? in post-navigation; class=post-xy)

Hello
Recently i made a child theme out of 2013 theme. Mostly everything works fine. However there are a couple of issues that i am stuck with.
First:
On https://genericons.com/#mail i have detected all genericons and there appropriate reference number.
But i cannot find the origin of the arrows that are displayed when navigating between post (previous/next). I’ve searched the css meticulously, but i can’t find any clue where these arrows are coming from.

Second:
In my style.css i have used the ?post-xy? id to change aspects of certain pages. For instance the background-color or the font color. But with certain elements like the ?post-navigation? class this method has no effect. Is the ?post-xy? limited to certain areas or styles? Is there a way to have page-specifically more design-control?

Thank you all for your time and interest.

regards
theo