I have a very basic understanding of code but am not a developer. I’d like to understand how theme code is laid out to make very simple changes without having to hire a developer. Is there a guide?
The first thing is to familiarize yourself with the Developer Tools in your Browser.
This is how you can find what element you want to change, how to select it for CSS rules and try simple edits.
Shopify has a reasonable documentation on Liquid, HTML and CSS tutorials are widely available on the net and on Youtube.