[thincom2000]

I doubt your white on white "Posted by:" has anything to do with combining alt1 and thead classes since that would just mean one of those classes is foolishly white on white itself.

This is likely caused by a conflicting setting in the Usergroup Manager:

Username HTML Markup (Specify an opening and closing HTML tag with which to display the names of users in this group. This applies to names that appear on Logged-in Users, Who's Online, Users Browsing Forum, Users Browsing Thread, Posts, Member List etc.)

If this is set to white, that's why usernames are appearing that way. A template edit to change $post[musername] to $post[username] would fix this problem.