The <html> element can be referred through the document.documentElement property. In general, the root element of any document can be referred through .documentElement.
document.documentElement.style.background = '';
Note: .style.background returns the value for background as an inline style property. That is, defined using <html style="background:..">. If you want to get the calculated style property, use getComputedStyle:
var style = window.getComputedStyle(document.documentElement);
var bgColor = style.backgroundColor;