Document what CSS is available for overriding

### What is the problem or limitation you are having?

If I don't have a good way to turn on web dev tools (since the instructions are Mac-specific on how to do that), I don't know how I'm supposed to know what CSS classes I can override.

### Describe the solution you'd like

Either documentation on what CSS classes are available or more instruction on how to infer it.

### Describe alternatives you've considered

Maybe there's a way to load the presentation in a browser directly?

### Additional context

_No response_

Review the existing documentation and the Mac-specific instructions for enabling web development tools first. Document the CSS classes available for overriding or explain how to discover them, and clarify whether the presentation can be loaded directly in a browser.

Written by the indexing model from the issue text.