Warning: Cannot modify header information

I recently moved my blog to a new host and now get this message when I try to login on the wp-admin screen:

Warning: Cannot modify header information – headers already sent by (output started at /home/sh1tfuck/public_html/wp-content/themes/rockwall/functions.php:84) in /home/sh1tfuck/public_html/wp-includes/pluggable.php on line 865

I don’t even get the option to log in! I have taken one space out of the end of the wp-includes/pluggable.php file, but this hasn’t made any difference.

Any help greatly appreciated!

(ps – Pardon my choice of directories above, I was having issues with my host at the time!)