The text in the cells of a head row do not get wrapped in a "p" element. Therefore, you need to style the "th" element inside of the "thead" element. This style is missing from your stylesheet.
You'd need something like:
thead th.tableblock {
font-family: "Open Sans", "DejaVu Sans", sans-serif;
}
otherwise, it inherits from body (which is set to Noto Serif in your stylesheet).
Cheers,
-Dan