The CSS below will be stored in the URL as a query string. This BBS doesn't set HTTP cookies and won't remember you next time you visit. So the only way to store this setting is to save that URL.
This site doesn't use Javascript. If you know what you're doing you can take the responsability to inject some yourself with extensions like Greasemonkey or Tampermonkey. Below are some examples to get you started: an extension for syntax highlighting (uses highlight.js) and a simple word filter to replace profanities with what you want.
zge wrote a slick mode for posting from Emacs, let's all M-x sbbs: sbbs.el
There are Fossil repositories for SchemeBBS and all the user-contributed code