Where are the WP Built-In Gallery codes?

Hi,

I’ve been trying to style my wp built-in gallery, but i cannot find where are the codes ?

looks like every galleyr pages have this text:

” This entry was posted on Thursday, April 1st, 2010 at 5:55 pm and is filed under NEWS. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site. Edit this entry.”

Where can i edit it ? I want to add my own css into it

would love some help on that, thanks