PKaBoot (Programmable KaBoot) is a comparatively small userscript that allows one to map most keys on their keyboard to the answer buttons on Kahoot. By default, the script will map the 1, 2, 3, 4 and Space keys to the red, blue, orange, green and Submit buttons (respectively), but most other keys can also be assigned using the in-game menu.

Note for clarification: The Submit button is shown as black on the UI and is used to submit multi-select answers.

Another note: The default keybinds are not really ergonomic but are used because other quiz systems (such as Quizizz) have these by default. I personally recommend using A, S, D and F for the main answer buttons instead.