two datasets, two background colors

Hello, I am using the "change datasets" example with the grid widget. I'd like to have one aw-alternate-odd background color for one dataset, and another color for the other dataset. I'm trying to update the css property through javascript when the datasets are changed, but it does not work:

document.getElementByTagName('aw-alternate-odd').style.background = " #9cf";

Any ideas?