Custom Icons aren't officially supported with the NightLight screenshot recognition system yet. It's currently only trained on the default icons - packs which are similar may find some success but it isn't guaranteed.
Recognition of custom icons has always been requested but the effort involved never quite made sense compared to the number of users interested in it.
When the Icon Toolbox shut down, I felt like I was in a unique position with NightLight to offer a replacement. As awesome as it is to step in to support that part of the community, it does mean we've ended up in a position where custom icons are so closely tied to the site despite it not being able to recognise them.
Fortunately, this integration has solved one problem - I now have a database of custom icons (which is naturally kept updated) for the system to learn from (yay).
Building a system that can recognise them at least moderately well is a task that's going to take many hours though it's one I'm looking forward to. I'm not sure when it'll be completed as there's a lot involved, but I appreciate your patience in the meantime.
If you want to support the development of the project, please consider supporting on Ko-fi - the more support I get, the more time I can work on the project and features like this.