Revert "Simplify variables in compile-stylesheets"

This reverts commit f069acc1dad00797465ecc55d0e21109dfc218e0.

Commit f069acc1 changed the way that the Kuma root was detected in
compile-stylesheets so that output from Stylus would be less verbose.
The particular implementation, however, introduced a bug where the
script could not be run from within its own directory. As it turns out,
there is no easy, portable way to do what commit f069acc1 intended.
Merge pull request #2566 from openjck/revert-f069acc1

Revert "Simplify variables in compile-stylesheets"