Here's how to remove border lines:
(This tutorial is for the Minima template but it may be similar for other templates.)
1. Go into the Design tab and click "Edit HTML".
2. Always back-up your template before you make any changes to your HTML. Read here to learn how to back-up your template.
3. Scroll down until you see this:
Where you see "border: 1px", change this to say "border: 0px", like this:
Now do the same thing in a few more places, like here (for the second border line around the header):
and here (if you don't want a border around your profile picture):