Elgg don't have css , the css is handled dynamically by elgg.

the css folder is at:
/views/default/css/

unless you are using a non-default theme, in which case the css folder will be the same type of path as i wrote here - contained within the theme folder.

I'd recommend to use a theme or write a new one to make css changes. Don't write over core files.

Rodolfo Hernandez

Arvixe/Elgg Community Liaison