How to get comment id in callback function?

The callback is already passed arguments but crazily the codex doesn’t document them. However it does give you an example and the first two lines are:

function mytheme_comment($comment, $args, $depth) {
    $GLOBALS['comment'] = $comment;

which matches the equivalent functions in the bundled styles. Once you’ve set the comment object passed as the global as in that example then you’re then free to use get_comment_ID() as usual.

If you didn’t want to set the global then

$my_comment_ID = apply_filters( 'get_comment_ID', $comment->comment_ID );

is equivalent, but it’s probably safer to go through get_comment_ID().