Usually CSS is used on websites. Does ubuntu got an "CSS interpreter" running or something like that? And why was CSS choosed and not something else (I assume windows and mac use a different theming format)?
The answers to your question are all contained in the link you posted!
- Unity uses GtkStyleContext for theming which uses CSS for pretty much everything
- This was chosen because it's faster and has a more simple architecture than the old Metacity engine via gtk-window-decorator
- GtkStyleContext does the interpreting and rendering of the CSS (via GtkCssProvider)
Presumably CSS was used not just because there are fast interpreters for it but to avoid inventing something new unnecessarily.