Hello AS Team

In the topmenu of this template is a small arrow shown if you add some submenues to one of the links. I think this arrow isn't very nice looking ;) and it would be great if I could change this into something else - maybe even some sort of image or so.
Is this possible and where do I have to change this?
I thought it is the "list-item" but I tried everything to find where to edit this. Without success.

Can you help me with this?

Best regards,