It does not work because when the $("#twitter_feed ul").find("li:first")
is executed, no tweet has been loaded, so no li
exists. You should use a callback function, if I do understand twittie :
$('#twitter_feed').twittie({
dateFormat: '%b. %d, %Y',
template: '{{avatar}} {{tweet}} {{screen_name}} {{date}}',
count: 3
}, function () {
$("#twitter_feed ul").find("li:first").css({"color":"red","border":"2px solid red"});
});