Unfortunately Theme URI is not available via default get_theme_data(), which is probably for the best because I don't want to conflict with standard wordpress conventions.
Couldn't you do:
add_action( 'extra_theme_headers', 'frosty_extra_theme_headers' );
function frosty_extra_theme_headers( $headers ) {
$headers['Github Theme URI'] = 'Github Theme URI';
return $headers;
}
Then you can call:
$theme_data = get_theme_data( '/style.css' );
$theme_data['Github Theme URI']