Looking at the sources of these two pages:
https://textboard.org/prog/
https://textboard.org/prog/?css=2ch
It's clear that the main difference between them is that a different stylesheet has been requested, and that all internal links preserve the value of the css variable.
When testing this feature on my own instance, this feature doesn't work, and the hard-coded value of default.css in the main-template procedure is always requested.
I was considering changing main-template to take the css parameter as an additional argument, but that won't help with the internal links. Does anyone know how the internal links are modified on this site?