Are there any themes which are free that look slick and modern, that people can think this is a pretty state-of-the-art website while fully compatible with 2.3.1 (=in most cases up to date by developer)?
Welcome to the community.
Your question, and any responses, will be entirely subjective.
Best thing is to try themes on a test install and see what you like.
Themes by vrijvlinder are well supported.
With a bit of time and effort, and a working understanding of CSS, Vanilla is pretty easy to customise any way you want.
@whu606 Thank you very much for your response. Then which of @vrijvlinder 's themes would anyone recommend starting on subjectively? It's because it's pretty hard to search the addons based on the author.
Look at her profile add ons link and then just do a simple browser search on the term "theme".