mod_rewrite filename-123.css to filename.css

.htaccess

# only for files / directories that don't exist
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d

# remove "-" and any digits preceding a file extension
RewriteRule ^(.*)\-[0-9]+(\.[a-z]+)$ $1$2 [L]

# works for CSS, JS, images, etc.
# means you can set far-future expires headers (so browsers cache files lots)
# whilst being able to easily invalidate the cache, just by changing the version number