You want to change the string that says "Those Beyond Redemption"?
If i remember correctly, the game handles the headers of bonusobjectives and world quests not as headers, but as lines. If you change
Lua Code:
line 215 - line.Text:SetFont([[Fonts/ARIALN.ttf]], 12)
to something else, it should change the bonusobjective header also. You could add a check for textcolor and only apply your headerfont on yellow.