I think this should be core - as we also have a best answer selected class (.qa-a-count-selected) for answers:
// override function q_item_title in advanced theme to mark closed question
// check for closed question, ignore closed news of eetv
$closed = @$q_item['raw']['closedbyid'] !== null;
// add CSS qa-q-item-closed to question title
'<DIV CLASS="qa-q-item-title'.($closed ? ' qa-q-item-closed"' : '"').'>',
Then you can use css to target the question with .qa-q-item-closed a to assign another color.
Thanks @Scott btw.