You can amend your CSS this way:
font: 100 1em/1.5em 'Lato';
font: Ultra-Light 100 1em/1.5em 'Lato';
font: Ultra-Light 100 1em/1.5em 'Lato', sans-serif;
Also, make sure you have included the 100
weight of the Lato font, which is not included by default. If you need, the URL is:
http://fonts.googleapis.com/css?family=Lato:100,400