Starter Theme:
Check Underscores or Sage for starter theme.
Underscores is pretty basic, but a very good starting point. On the other hand, Sage has more options compared to Underscores, but you may need a bit more time to get used to with all the dev tools (if you are not already familiar with modern dev tools).
CSS Framework:
For CSS framework, try Twitter Bootstrap.
Theme Development Guide:
Follow The Official WordPress Theme development guide for customizing a starter theme or creating a new theme from scratch. You may also check the Old, but useful Codex document on WordPress theme development.
Also, there are some q&a gold mines here on WPSE as well.
Gutenberg: As WordPress core team is set to change the default editor to Gutenberg in the near future, it’s a good idea to familiarize yourself with Gutenberg as well. This Official Guide can be a good starting point to learn about Gutenberg.
(Thanks WebElaine for reminding this!)
Learning Curve:
I’d suggest you to learn basic theme development with WordPress first, then become familiar with CSS Framework, Gutenberg etc. Otherwise it’ll all seem like too much information.