How do I edit the format of the Gridmarket side bar category list?

I would like to remove the green arrow graphic from the side bar category list, and move the listed items over flush left.

I found the graphic, so I could replace it with another image, but I rather remove it from being used in lists.

My custom.css style-list is set to none. I looked in sidebar.php etc. and could not find the link to the graphic. This reference must exist somewhere.

Overall, any changes I do to header.php and sidebar.php - will they be overwritten during a theme update?

Thank you.