Answer 1

document.getElementsByTagName() returns a collection of elements like an array not a single element. You need to select it's contents as you do in an array like:
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var elems = document.getElemenetsByTagName('body');
elems[0]; // First body element.
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Use document.body to select the body element.

Answer 2

For accessing body element, use document.body

To set background color of body
document.body.style.background = 'black';

Answer 3

Can you please post the code here?