[Adam Harley (Kawauso)]

The ID of the entire first post (including the poster’s details) would depend on your theme’s code, but in Kakumei it’d be div#position-1. If it’s anything else, it’d be under post.php, but you could just use Mozilla Firefox’s DOM Inspector or Google Chrome’s Inspect Element to check the ID on the page anyway.

Again in the case of Kakumei, the post itself is .threadpost, so you could use something like <style>div#position-1 .threadpost {background-color:#FFFF99;}</style> to change the background colour.

Hope that’s some help