While it is not convenient or what I’d consider a long-term-viable fix, the fact that Station gives access to View -> Developer -> Toggle Page Developer Tools does mean it’s possible to inject custom styles from the console. At least, I have that working for my dark Slack theme.
However, since I haven’t figured out a way to splice it into Station itself somewhere, I have to do it every time I start Station for every instance of every app I want to do it for. I just thought I’d mention it as something to explore for anyone else coming across this ticket who considers it worth the effort. If anyone can work out where Station might have a file we could edit to splice in a DOMContentLoaded event listener to an app as it’s being fired up (and how to tell which app it is), that might get us at least to the same level of convenience as loading a custom theme in the Slack desktop app.
Stylish support, or any kind of built-in setup for custom scripts or styles, would certainly still be a better long-term solution.