Attempts to identify slack installs on macOS and linux and inject code to load custom css.
Run ./install.sh [theme-name] [path-to-slack-resource-dir]. Theme name defaults to default, and if the slack resource path is omitted it will search known locations for installs. If it can't find it, you will need to supply the directory that ssb-interop.js lives in (on my linux system, for example, this is /usr/lib/slack/resources/app.asar.unpacked/src/static/ssb-interop.js
Rerun the install. It will either reinstall if ssb-interop.js was overwritten, or it will detect that the shim is already in place, restore the backup file, and re-apply.
Run ./uninstall.sh [path-to-slack-resource-dir] to restore the original version of ssb-interop.js. Will only run if it detects an altered ssb-interop.js, so as to avoid overwriting an updated version with an old one.
Due to "security reasons", Electron can only load 3rd party content via fetch/XHR over HTTPS. Therefore, you need to host any css you have somewhere. By default, the themes file pins to specific revisions on github. If you want to customize, change the CSS and add your own theme.
Default theme is slack-night-mode from https://github.com/laCour/slack-night-mode