WordPress plugin add_filter problem?

Hello,I’m begginer in php and wp programing.I want to filter posts,for example,just on archive.php page I want to do this,if some post contains word girl,I want that post to set up red background for that post,if not then background will be blue.I know to do this directly from code,to change archive.php but how can I do this from plugin.I know basic step how to wrote a plugin.
Can someone help me please.