How did you do this ?
Think you need to check the .css files.
There is a modification for phpBB3, called rounded-borders.
The stylesheet of that modification holds the answer somewhere, when done incorrectly, such a gap came up.
It had to do with modifying overall-header.html with <div class=”outside”> etcetera, and then the <div class=”notopgap”> came.
Maybe the guys on can explain how they did it.
I’m very curious too.