I am having difficulty adding an HTML element to my menus on my WordPress website because I am not finding the SourceCode of my website to edit it. I am using WordPress with Sinatra Theme. Is it possible to edit my WordPress website source code and if not how can I add an element of HTML to my Navbar and add a CSS style to it.
You cannot add html to build in wordpress menu. First there is PHP, second you need to create separate menu in HTML and CSS and use it instead (if you really want). Also if you even find a way how to implement it make sure you create child element because every time when you update WP all changes will be gone.
The second option: You need to create entire header including menu. Switch off build in WP menu totaly and put new instead.