Alot of editors, such as Notepad++, Atom, etc have a 'Search in Path' function.
You can select a directory and perform a search for a work within that specific directory.
So you'll have to do that and search for either the HTML comment or the specific
div element, with its class.
Seeing it's a custom style, we can not know for sure, but the element you highlighted is in
navbar_header.html in prosilver. But once again, the style you're using is not prosilver so we can not know for sure in which file it is.