The theme looks way too different to the default look. I won’t be able to help with this one since I don’t know what exact changes were made. Are you the one who edited it? If someone else did the customization then maybe they would know more.
My advice is to start again with the menu customization. Copy the original Navigation CSS to your theme then edit it step-by-step (testing along the way) to see what might have cause the shift to the right.