Thanks Andi,
I can see this file on the server. I can also see some .css file in a root catalog, which may supersede it (although disabling it don't change how wiki looks...)
On the same server space we have webpage, booking system and now hopefully wiki. The webpage though uses some more obscure CMS and I wasn't the one who created/migrated and managed it before. Now I'm trying to understand it but it looks like a mess to me.
I may have to do it again from the scratch.
I really appreciate your help and hope to get this working ;)
EDIT
Server admin pointed me to this file:
http://www.nanofunction.org/wiki/lib/exe/css.php
It, apparently, doesn't resolve properly and doesn't give my wiki it's beauty! Any idea why could that be?