grep(1) -R --include='*.css' '.ad' .
print lines matching a pattern
-R, -r, --recursive
       Read all files under each directory, recursively; this is equivalent to the -d recurse option.
       Search only files whose base name  matches  GLOB  (using  wildcard  matching  as  described  under
grep searches the named input FILEs (or standard input if no files are named, or if a single hyphen-minus
(-) is given as file name) for lines containing a match to the given PATTERN.  By  default,  grep  prints
the matching lines.
source manpages: grep