Your starting point would be the documentation section on creating themes. A minimal Q2A theme consists of 2 files:
- qa-theme.php: The "entry point" for the theme. In this file you'd override the methods of the theme class, if needed.
- qa-styles.css: The stylesheet for formatting the pages.
I don't really have a recommendation for what to use as the base for a "Quora" theme. Something that already looks close to your desired result might take less time to complete but will likely earn you some legacy code, whereas starting with a clean slate will probably take at least somewhat longer than adopting existing code but will get you exactly the result you need.
I'm also not sure if Bootstrap will do you any good. I've never used it, so I don't know how well it would work in the context of Q2A.