The magnetic fields created by the earth are very powerful but spread out, kinda like a gentle breeze can push a boat, although the much faster and 'more powerful' air blowing out the front of a hairdryer or fan would never have a hope of pushing a yacht or boat along (even though it's faster).
The earth's magnetic fields behave in vaguely the same way. I'm sure you've see what happens when you hold a magnet up to a CRT display, all the colours go weird and the picture warps around the magnet slightly. Well it's the same thing with the earth's magnetic field. It's not concentrated enough to cause any discolouration or major geometry issues, but it can 'rotate' the whole screen if it pulls in the right direction.
In a CRT, a gun at the back of the tube fires cathodes onto a phosohor screen at the front of the CRT which lights up wherever a cathode collides with it. The cathodes are aimed at the right parts of the screen by elements on the left, right, top and bottom of the tube which change their charge to push and pull the travelling cathodes in the right direction and point on the screen.
The same happens with the earth's magnetic field, it pulls the travelling cathodes round and rotates the image.