> Reading material design from Google https://material.io is a great start. You may not like the UI of it, but the UX and explanations of why things are this way are really interesting.
To add, I've found reading the design documentation of some web and mobile UI libraries to be very enlightening. For example, the colors section of BlueprintJS has helped really narrow down how I use colors: https://blueprintjs.com/docs/#core/colors