Cool. Here are some good starting points.
A decent crash course in general UI design principles: https://medium.com/hh-design/crash-course-ui-design-25d13ff60962
Best practices from usability.gov: https://www.usability.gov/what-and-why/user-interface-design.html
Practice makes perfect, so here is a MOOC:
https://www.coursera.org/learn/ui-design
Color and accessibility: https://accessibility.umn.edu/core-skills/color-contrast
Grids and layout design: https://www.smashingmagazine.com/2017/12/building-better-ui-designs-layout-grids/
How to choose colors: https://www.smashingmagazine.com/2016/04/web-developer-guide-color/