overriding theme’s output code

just found this hidden little treasure plugin, but I have a problem outputting the icon and I don’t quite understand your solutions on the matter.
My theme has a “add icon” that pulls from media library and outputs it. I thought that your plugin would override that but it doesn’t. I should ask the theme developers but right now thy are out of business due to the Coronavirus, and I know it’s a pretty vague question, but do you have any suggestion for where to look?