Would you try out the steps I have pasted here? This works for me on macOS Catalina.

Thanks for the response - I followed the steps and installed it locally. The only problem I have now is that it the browser doesn't seem to be picking up the settings.toml file, and just launches blank. I added ~/.config/castor/settings.toml with the settings in your castor_settings.toml.example file, but they don't seem to be loading. Do I need to tweak something in Castor to connect it to the settings.toml file?

Huh, you're right. For some reason it isn't doing anything with the settings file. This is probably something to do with the difference in directories with macOS. I'll see if I can figure it out when I have the time. If anyone else has any input on this, it would be greatly appreciated.

Instead of using ~/.config/castor/settings.toml, see if a settings file exists at ~/Library/Preferences/castor/settings.toml.

Yes, that solved it! Thanks so much for your help, I'm trying out editing the theme now.