My SchemeBBS doesn't allow me to change CSS at /board/preferences. Does SchemeBBS code change? I'm running the code at https://gitlab.com/naughtybits/schemebbs
There are threads about this on /prog/:
https://textboard.org/prog/39
https://textboard.org/prog/140
It looks like it is some ngix magic:
https://textboard.org/prog/39/26
Unless that changed since.