Sure, you can either just change the class or add the css for the color inline there.
If you just want to change the background inline, instead of adding a new class, just go:
Code:
<td class="alt2" id="td_post_$post[postid]" style="background: #00ff00 url('smiley.gif') no-repeat fixed center;">
(All just a thrown together example - don't use that code cuz I have no idea what you would produce!) Doing so for the font color would be similar.