Go to "User CP" (psst, it's next to "Home", beneath the pretty Gossip Rocks banner).
Select "Edit Options" from the left vertical bar.
Scan down to "Miscellaneous Options"
Change your 'Message Editor Interface' to "Standard Editor" - the default WYSIWYG editor isn't compatible with every browser. You'll lose some of the smilies and other fun options, but the message box will change from yellow to white.
Cheers, happy posting.