This is a simple, quick fix - how to remove those lines around your header. Especially useful if you have a fancy header with pictures and custom fonts.
Go to Layout > Edit Html and save a copy of your blog before you start (download it to your desktop). It only takes a second.
Click CTRL+F on your keyboard to bring up the Find box and copy&paste this code: #header-wrapper
into the box, which will bring up the section you want, highlighted in green.
Change the border: 1px to border: 0px in both sections shown, and that's it - job done!
Of course, if you want a thick border you just do the same - but in reverse. Change the border: 1px to border: 6px, or whatever thickness you want. Preview first, then save.