Within all the functionality of the css.php the script uses two
constants
DOKU_TPL and DOKU_TPLINC. Due to no possibility of changing these
constants at the proper time
Here's how you'd go about accomplishing this:
http://tatewake.com/wiki/projects:multitemplate_for_dokuwiki#how_to_modify_your_templates
I've not peeked into css.php before, but the idea is the same. If you
modify $DOKU_TPL or $DOKU_TPLINC before the file is loaded, you get
your values, else you get the default. (And this works whether or not
your replacement is called or even exists.)
But you'd not only have to do this with css.php, but with your
template files as well.
Modifying css.php in this way would probably fix the problem with
style.ini I have as well.