I sure do like me a nice background theme! Whenever you start to realize that holy Hell I’ve been sitting in front of a computer for hours and why am I not outside enjoying the wonderful San Francisco weather — that is when you need to tune into a nice background theme and trick your mind into thinking you are on the beach or up in the mountains or playing Angry Birds?! Who the hell thought that was a good theme to make?
In any case, go check out the panoramic themes especially made for multiple monitors. Download and pick your favorite!
Then, make sure you go to control panel -> appearance -> personalization (you should be here already if you just downloaded and installed a theme), then click “Desktop Background” at the bottom and at the bottom of that settings page, make sure “Choose a fit” is set to “Span”. Note that you will need to do that for each theme since it doesn’t apply to all at once.