Since Design Studio is based off mostly UI5 components, here's a good starting list:
List of Supported CSS Classes - SAPUI5: UI Development Toolkit for HTML5 - SAP Library
Also check out the UI5 Theme Designer:
UI Theme Designer Developer Center
Short of these, download the UI5 library and you can go look at the different Theme files (I believe they are in LESS or CSS format)
Note, that Design Studio sometimes does override the out-of-box styling, I'd recommend finding the CSS file they add to the page at runtime using Fiddler or other F12 debugging tools. It'll be a .CSS file with something like 'zen' in the name.