Inline “Theme Icons” php file location

Hi,
Happy blocksy pro customer here and no complaints!
I’m looking to output custom svg icons in mega menus.

Closest thread I found on icons is this:
https://www.ads-software.com/support/topic/change-card-icon/

Since some inline “Theme Icons” are output in menu item settings, which php file can I modify and save in my child theme to add SVG icons of my own?

Thanks a ton!