Global Roboto with Open Sans Headers user style
This is a global style that changes every font everywhere to be the Roboto font, or the Open Sans font in the case of Headers. This may include the browser itself, which may or may not be good.
It requires the Stylish extension, which is available for Firefox and Chrome. It also requires the Roboto and Open Sans fonts, of course.
Download/installation at: http://userstyles.org/styles/100206/global-roboto-with-open-sans-headers