Promo codes can already only be entered by logged in users. A non-logged in member would be able to press the sequence of keys to display the alert, but they probably would not know to, nor is there any harm in letting them...?
To change the text in the alert prompt, simply follow the instructions already given and replace the word
'yay!'. Alert prompts cannot be styled in any way. If you want to pop up a div and have it styled with css instead, learn Javascript or jQuery. Or, if you're using Bootstrap already, perhaps install and learn to use
Bootbox.js.
If you want to play a sound,
here's the top result I got from Google.