I’ve had issues in the past where I couldn’t get the icon to display after adding it to appicons.svg due to it being out of the viewport that the theme uses. The viewport is kind of like a cropped view, so if your image doesn’t fall into that part of the image file you won’t see it. I’ve also had the old version before the addition being stuck in the browser cache, so clearing the cache might just be the ticket too.
If you open the file in a vector editor like inkscape helps a lot with troubleshooting these kinds of issues, you can then see if it’s showing on top of the pile of other icons and if not you can drag and resize as necessary in a visual way. I always also work on a copy of my appicons.svg file when doing this just in case I accidentally mess something up.
Edit: @rlkoshak, OP is referring to this if you are curious: Matrix theme for Habpanel