Berlin Theme HTML file location

I am trying to make CSS and HTML edits to my Berlin theme. I have located the CSS folder/file, but I cannot seem to locate the html folder/file. Specifically, I am trying to customize the homepage with new div types. Am I going about this process correctly? If so, how do I access/edit the html?

I have located the html file under the index.php. However, now I am realizing that I do not know the syntax for customizing the new div I've created. I've successfully created my div to display on my homepage, but I cannot have it call on a simple page I've created to display information. Can you suggest some basic php resources to get started on learning the language for some basic customization?

Besides general PHP references like, you'll likely want to get familiar with Omeka's documentation. In particular, this guide to modifying themesL will likely give helpful guidance.