Hi and welcome, y8sy!
Wonderful! We're glad you're finding some success going through the class!
[part 1]
1) Edit the file to see what it contains. Did you download the template project files?
www.ostraining.com/media/files/cooking-with-kids-template.zip If so, you'll find the joomla.css file within the template's CSS folder.
2) It's not that it's exactly specific to Joomla, but rather just formatting that you might not change from Joomla install to Joomla install. For example, formatting for System messages.
3) No, you can create your own styling if you'd like. You can do it however you'd like.
[part 2]
Have you had a change to go through our CSS class yet? That should be able to help you get the styling that you'd like for your menus. Here's a direct link to the class:
www.ostraining.com/courses/class/coding/css/view/ Hope this helps!
Kind regards,
Nick