Theme Checker Text Domain

_x() is for string with a context. So your second argument is just the context, and there is no text domain.

Suggestion:

$string = _x( 
'%1$s %2$s - %3$s %4$s',                # string to translate
'recent comments 1 = gravatar, 2 = … ', # context for translators
'thst'                                  # text domain
);

$visible = sprintf( $string, get_avatar( $comment, 48 ), … );

$output .= '<li class="recentcomments">' . $visible . '</li>';

Explain all numbers in your context parameter.