This, like other github themes, has bad CSS that makes the monospaced text come out as serif on Linux. I know the github crew uses Macs but it makes me a little sad to see they don't test on the system that spawned their namesake. :(
code {
font-family: "Lucida Sans", Monaco, Bitstream Vera Sans Mono, Lucida Console, Terminal;
}
For BODY or HTML (whichever contains my global font-family rule) I finish off the list with "..., sans-serif".
I've never had amazing luck w/typography outside of Mac/Win, but like you point out, its poor style to leave everyone else without even a base style to work with.