The line that has the .controls li a changes the Create Thread button on the Thread List page. The line that has the h2 should change the title bar text on the Posting page. Just change the "What's on Your Mind?" to the text you want - but be sure to keep the quotation marks in both instances.
REPLACE the old code I gave you with the above - putting it in the FOOTER for the board to be affected (Admin > Structure > Headers & Footer > (select board) > Footer).