Robb. Knight.

MacOS customisation is basically dead. Plenty of apps that used to work, no longer do. You can still have custom folder icons which is nice but I wanted custom sidebar icons for those folders, like the developer one.

I started googling around trying to find something for recent versions of MacOS and the only thing I found was this Reddit post outlining a method to do it with PNGs. Between me and Keir we were able to make it work not only with any standard SF Symbol but custom made ones as well.

You'll need Xcode installed to do this but the instructions relatively simple. I will say it's really flaky when building, sometimes it works, sometimes it doesn't, sometimes it just shows a random icon instead. All of the instructions are on the GitHub repository including some dandy screencasts from Keir.