rgrep.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.

Other Options
    --line-buffered
           Use line buffering on output.  This can cause a performance penalty.
--include=GLOB
       Search only files whose base name  matches  GLOB  (using  wildcard  matching  as  described  under
       --exclude).
source manpages: rgrep