I think your regex is not correct, update it as follows
Code:
$HTTP["url"] =~ "\.(jpg|gif|png|css|js)$" {
expire.url = ( "" => "access 7 days" )
}
Reload lighttpd again:
Code:
service lighttpd restart
curl -I http://forum.topflood.com/themes/forums/style/style.css | grep Cache
You also need to Etags, if you are not using load balanced setup by adding following lines:
Code:
etag.use-inode = "enable"
etag.use-mtime = "enable"
etag.use-size = "enable"
static-file.etags = "enable"